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3.12.3: Three phyla of plants - Biology

3.12.3: Three phyla of plants - Biology


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As animal phyla differ by body plans, plant phyla differ with life cycles. One of reasons is that plants do not move, and young sporophyte always starts its life on the mother gametophyte.

Mosses have sporophyte which is adapted only for spore dispersal. Gametophyte is then a main photosynthetic stage which makes most of photosynthesis and therefore need to be big. However, it cannot grow big! This is because for the fertilization, it needs water. Therefore, mosses could not be larger then the maximal level of water.

To overcome the restriction, stages role must reverse. Ferns did that, and their gametophyte is really small and adapted only for fertilization. This works pretty well but only if the plant body is relatively small.

Trees capable to secondary growth (that is, thickening of stem with special “stem cells”) will experience the ecological conflict between ephemeral, minuscule gametophyte and giant stable sporophyte. Whatever efforts sporophyte employs, result is unpredictable. Birth control, so needed for large organisms, is impossible. One solution is not to grow so big.

Another solution is much more complicated. They need to reduce gametophyte even more and place it on sporophyte. And also invent the new way of bringing males to female because between tree crowns, the old-fashioned water fertilization is obviously not possible. To make this new way (it called pollination), some other external agents must be employed. First was a wind, and the second came out of the clever trick to convert enemies into friends: insects.

Still, result was really cumbersome and the whole life cycle became much slower than in ferns, it could span years! The only way was to optimize and optimize it, until in flowering plants, it starts to be comparable and even faster then in two other phyla.


4 Types of Plants (Kingdom Plantae)

Plants are classified mainly based on vascular tissue and reproductive tissues. Plants that lack true roots, stems, and leaves due to the absence of vascular tissues are placed under Phylum Bryophyta. Plants that have vascular tissues for the transport of food and water are commonly known as tracheophytes.

In the two-kingdom classification scheme, ferns and seed plants are grouped in Phylum Tracheophyta. Tracheophyta comes from the Greek word tracheis which means windpipe and phyton which means plants. The name pertains to xylem tracheids. There are about 212 000 species of vascular plants. In the five-kingdom scheme of classification, the major groups of tracheophytes - the ferns, cycads, conifers (pines) and flowering plants are elevated to phylum level as follows:

1. Phylum Filicinophyta (Ferns)
2. Phylum Cycadophyta (Cycads)
3. Phylum Coniferophyta (Pines)
4. Phylum Angiospermophyta (Flowering Plants)

These plants are similar in their vascular tissues, chlorophyll, and their bodies are differentiated into true roots, stems, and leaves.

Diagram for the different types of plants


The Current System

As scientists learn more about organisms, classification systems change. Genetic sequencing has given researchers a whole new way of analyzing relationships between organisms.

The current Three Domain System groups organisms primarily based on differences in ribosomal RNA (rRNA) structure. Ribosomal RNA is a molecular building block for ribosomes.

Under this system, organisms are classified into three domains and six kingdoms. The domains are

  • Archaebacteria (ancient bacteria)
  • Eubacteria (true bacteria)
  • Protista
  • Fungi
  • Plantae
  • Animalia

Salient Features and Classification of Plants Class 11 Biology

Plants are unique in their physical appearance, structure, and physiological behaviour. Apart from that, they also vary in their habitats, tolerance to the environment, and nutrient requirements.

Biologist Whittaker gave the Five Kingdom Classification, for all the living organisms. He divided the living organisms into five kingdoms – Protista, Monera, Fungi, Plantae, and Animalia. In this topic, we will learn about the special features and classification of Plants or Kingdom Plantae.

KINGDOM PLANTAE

During the early times, the biologists used the apparent features of plants and classified them based on these features. The morphological features included colour, number, the shape of leaves their habitats, etc.

This system of classification was an artificial system of classification because the plants were classified based on their physical characters. The vegetative characters are susceptible to changes because of the effect of the environment. Therefore, many closely related species were classified under different divisions.

Gradually, biologists became more aware of the other characteristics of plants, so they slowly started another system of classification, called the natural system of classification. This system considered the external and internal features of plants for classifying them.

According to the new system of classification, the Kingdom Plantae has been divided into five major groups. They are:

Botanists have grouped the plants into two major groups: non-vascular and vascular. The former includes the early plants without having a vascular system. The latter (vascular plants) consists of all the members who had developed a vascular system. Vascular system, in plants, is an assembly of conducting tissues and associated supportive fibers. Xylem tissue is a conducting tissue that transports water and dissolved minerals to the leaves. Phloem is a tissue that conducts food from the leaves to all parts of the plant. The commonly used plant classification system is:

NON-VASCULAR PLANTS

The nonvascular plants do not possess any vascular tissues that can help them transport water and nutrients. These types of plants are considered to be the earliest living plants on the planet. The non- vascular plants include:

1. Algae: They are plant-like organisms that are usually photosynthetic and aquatic, but do not have true roots, stems, leaves, vascular tissue and have simple reproductive structures. Some authors categorize the microscopic, unicellular green algae (Division Chlorophyta) in the Kingdom Protista, and the larger ones, multicellular green algae (Division Chlorophyta) in the Kingdom Plantae. Algae are ubiquitous in nature,they are found in the sea, in freshwater andmay be found everywhere in moist conditions. Most algae are microscopic, but some are very large, e.g. some marine seaweed which exceeds 50 m in length. The algae possess chlorophyll and can manufacture their own food through the process of photosynthesis.

2. Bryophyta

i. They are the most diverse group with more than 10,000 plant species. They constitute the simplest plant phyla.

ii. This phylum includes mosses, liverworts, and hornworts. Regarding physical appearance, mosses are small and inconspicuous.

iii. Bryophytes do not have vascular tissue and any wood that can give structural support to them.

iv. They lack true leaves, stem, and roots that can help them transport water and nutrients. Hence, they grow in a narrow range of habitats.

v. Bryophytes play an important role in lessening erosion along with the water bodies, carrying out water and nutrient cycling in forests, and regulating the temperature in permafrost (a thick subsurface layer of soil that remains below freezing point throughout the year, it occurs chiefly in polar regions).

vi. They are closely related to lichens (the symbiotic relationship between a fungus and algae) if compared to their habitats and physical structures. Like, both of them utilize the moisture in the environment to transport minerals and nutrients.

vii. Bryophytes live in moist places and have adapted to several methods that can help them bloom in dry periods. They reproduce through spores.

VASCULAR PLANTS

Vascular plants possess vascular tissues (xylem and phloem) that aid them to transport water and minerals throughout the body of a plant. This group includes members of the Phylum Pteridophyta, Gymnosperms, and Angiosperms. All these are classified as vascular plants. The different plant phyla are described below:

1. Phylum Pteridophyta

i. It is composed of almost 12,000 (with over two-thirds are tropical) species of true ferns and its allies.

ii. Pteridophytes are seedless plants. This classification of plants produces spores that are located beneath their leaves and are known as sporophylls.

iii. Pteridophytes can propel their spores even at long distances because of the spring-like structures called sporangia, which contains spores.

iv. There is no single characteristic that can describe pteridophytes because they are extremely diverse. Leaves of ferns are called fronds, which are naturally coiled until they unroll at maturity. They have horizontal stems called rhizomes and have simple leaves roots. As they are vascular plants, they are capable of transporting fluids.

v. With time, pteridophytes have already adapted to a wide range of habitats, like they can be aquatic, terrestrial, etc. but most of them prefer to thrive in tropical regions.

2. Gymnosperms

i. As compared with other plant phyla, gymnosperms include the tallest, the thickest, and the oldest living plants. They are widely distributed on the earth but dominate the temperate and arctic regions.

ii. Members of this phylum include pines, hemlocks, firs, and spruces, all the members are characterized by having wood, and green needle-like or scale-like foliage.

iii. “Gymnosperm” literally means “naked seed “.The members of this phylum are characterized by having cones (strobilus, plural: strobila), for reproduction instead of seeds.

iv. Gymnosperms are considered to be heterosporous. This means that they produce two different types of cones one for the male and female. The male cones are usually small as compared to the large cone of the female.

v. Members of this phylum are good sources of wood and paper. Apart from that, they provide food and habitat for animals, and these animals in exchange become important for gymnosperms in the dispersal of their propagules.

Some examples of gymnosperms are:

3. Angiosperms

i. They are also referred to as the flowering plants, and they are the most diverse plant phylum with at least 260,000 living plant species.

ii. Angiosperms show a vast diversity of plants. This includes trees, herbs, shrubs, bulbs, epiphytes (parasitic plants), and plants living in both marine and freshwater habitats.

iii. The largest families in this phylum are the Orchidaceae (family of orchids), Asteraceae (family of daisies), and Fabaceae (family of legumes).

In addition to the diversity, the members of this phylum exhibit several distinguishing characteristics like

a. Ovules/seeds that are enclosed within the carpel/fruit.

b. Double fertilization, takes place in angiosperms. Double fertilization is a complex fertilization mechanism of flowering plants, which involves the fusion of a female gametophyte (megagametophyte, also called the embryo sac) with two male gametes (sperm). Of the two sperm cells, one sperm fertilizes the egg cell, and forms a diploid zygote the other sperm cell fuses with the two polar nuclei, to form a triploid cell that develops into the endosperm. The part of a seed that acts as a food store for the developing plant embryo is called endosperm. It usually contains starch, protein, and other nutrients.

c. Male reproductive tissue is composed of two pairs of pollen sacs and many more.

iv. Due to their varied types, angiosperms serve as a wide variety of uses for animals, especially humans. Most angiosperms are good sources of food, medicine, clothing fibers, and wood.

Some examples of Angiosperm

Read another topic of class 11 biology on Kingdom Protista and its Characteristics

For School courses, Professional courses, and Skill development courses, Class 11 Biology Online Classes kindly visit www.takshilalearning.com


129 Bryophytes

By the end of this section, you will be able to do the following:

  • Identify the main characteristics of bryophytes
  • Describe the distinguishing traits of liverworts, hornworts, and mosses
  • Chart the development of land adaptations in the bryophytes
  • Describe the events in the bryophyte lifecycle

Bryophytes are the closest extant relatives of early terrestrial plants. The first bryophytes (liverworts) most likely appeared in the Ordovician period, about 450 million years ago. Because they lack lignin and other resistant structures, the likelihood of bryophytes forming fossils is rather small. Some spores protected by sporopollenin have survived and are attributed to early bryophytes. By the Silurian period (435 MYA), however, vascular plants had spread through the continents. This compelling fact is used as evidence that non-vascular plants must have preceded the Silurian period.

More than 25,000 species of bryophytes thrive in mostly damp habitats, although some live in deserts. They constitute the major flora of inhospitable environments like the tundra, where their small size and tolerance to desiccation offer distinct advantages. They generally lack lignin and do not have actual tracheids (xylem cells specialized for water conduction). Rather, water and nutrients circulate inside specialized conducting cells. Although the term non-tracheophyte is more accurate, bryophytes are commonly called non-vascular plants.

In a bryophyte, all the conspicuous vegetative organs—including the photosynthetic leaf-like structures, the thallus (“plant body”), stem, and the rhizoid that anchors the plant to its substrate—belong to the haploid organism or gametophyte. The male gametes formed by bryophytes swim with a flagellum, so fertilization is dependent on the presence of water. The bryophyte embryo also remains attached to the parent plant, which protects and nourishes it. The sporophyte that develops from the embryo is barely noticeable. The sporangium —the multicellular sexual reproductive structure in which meiosis produces haploid spores—is present in bryophytes and absent in the majority of algae. This is also a characteristic of land plants.

The bryophytes are divided into three phyla: the liverworts or Hepaticophyta, the hornworts or Anthocerotophyta, and the mosses or true Bryophyta.

Liverworts

Liverworts (Hepaticophyta) are currently classified as the plants most closely related to the ancestor of vascular plants that adapted to terrestrial environments. In fact, liverworts have colonized every terrestrial habitat on Earth and diversified to more than 7000 existing species ((Figure)). Lobate liverworts form a flat thallus, with lobes that have a vague resemblance to the lobes of the liver ((Figure)), which accounts for the name given to the phylum. Leafy liverworts have tiny leaflike structures attached to a stalk. Several leafy liverworts are shown in (Figure).



Openings in the thallus that allow the movement of gases may be observed in liverworts ((Figure)). However, these are not stomata, because they do not actively open and close by the action of guard cells. Instead, the thallus takes up water over its entire surface and has no cuticle to prevent desiccation, which explains their preferred wet habitats. (Figure) represents the lifecycle of a lobate liverwort. Haploid spores germinate into flattened thalli attached to the substrate by thin, single-celled filaments. Stalk-like structures (gametophores) grow from the thallus and carry male and female gametangia, which may develop on separate, individual plants, or on the same plant, depending on the species. Flagellated male gametes develop within antheridia (male gametangia). The female gametes develop within archegonia (female gametangia). Once released, the male gametes swim with the aid of their flagella to an archegonium, and fertilization ensues. The zygote grows into a small sporophyte still contained in the archegonium. The diploid zygote will give rise, by meiosis, to the next generation of haploid spores, which can be disseminated by wind or water. In many liverworts, spore dispersal is facilitated by elaters—long single cells that suddenly change shape as they dry out and throw adjacent spores out of the spore capsule. Liverwort plants can also reproduce asexually, by the breaking of “branches” or the spreading of leaf fragments called gemmae. In this latter type of reproduction, the gemmae —small, intact, complete pieces of plant that are produced in a cup on the surface of the thallus (shown in (Figure) and (Figure))—are splashed out of the cup by raindrops. The gemmae then land nearby and develop into gametophytes.


Hornworts

The defining characteristic of the hornworts (Anthocerotophyta) is the narrow, pipe-like sporophyte. Hornworts have colonized a variety of habitats on land, although they are never far from a source of moisture. The short, blue-green gametophyte is the dominant phase of the life cycle of a hornwort. The sporophytes emerge from the parent gametophyte and continue to grow throughout the life of the plant ((Figure)).


Stomata (air pores that can be opened and closed) appear in the hornworts and are abundant on the sporophyte. Photosynthetic cells in the thallus each contain a single chloroplast. Meristem cells at the base of the plant keep dividing and adding to the height of the sporophyte. This growth pattern is unique to the hornworts. Many hornworts establish symbiotic relationships with cyanobacteria that fix nitrogen from the environment.

The lifecycle of hornworts ((Figure)) follows the general pattern of alternation of generations. The gametophytes grow as flat thalli on the soil with embedded male and female gametangia. Flagellated sperm swim to the archegonia and fertilize eggs. The zygote develops into a long and slender sporophyte that eventually splits open down the side, releasing spores. Thin branched cells called pseudoelaters surround the spores and help propel them farther in the environment. The haploid spores germinate and give rise to the next generation of gametophytes.


Mosses

The mosses are the most numerous of the non-vascular plants. More than 10,000 species of mosses have been catalogued. Their habitats vary from the tundra, where they are the main vegetation, to the understory of tropical forests. In the tundra, the mosses’ shallow rhizoids allow them to fasten to a substrate without penetrating the frozen soil. Mosses slow down erosion, store moisture and soil nutrients, and provide shelter for small animals as well as food for larger herbivores, such as the musk ox. Mosses are very sensitive to air pollution and are used to monitor air quality. They are also sensitive to copper salts, so these salts are a common ingredient of compounds marketed to eliminate mosses from lawns.

Mosses form diminutive gametophytes, which are the dominant phase of the lifecycle. Green, flat structures with a simple midrib—resembling true leaves, but lacking stomata and vascular tissue—are attached in a spiral to a central stalk. Mosses have stomata only on the sporophyte. Water and nutrients are absorbed directly through the leaflike structures of the gametophyte. Some mosses have small branches. A primitive conductive system that carries water and nutrients runs up the gametophyte’s stalk, but does not extend into the leaves. Additionally, mosses are anchored to the substrate—whether it is soil, rock, or roof tiles—by multicellular rhizoids , precursors of roots. They originate from the base of the gametophyte, but are not the major route for the absorption of water and minerals. The lack of a true root system explains why it is so easy to rip moss mats from a tree trunk. The mosses therefore occupy a threshold position between other bryophytes and the vascular plants.

The moss lifecycle follows the pattern of alternation of generations as shown in (Figure). The most familiar structure is the haploid gametophyte, which germinates from a haploid spore and forms first a protonema —usually, a tangle of single-celled filaments that hug the ground. Cells akin to an apical meristem actively divide and give rise to a gametophore, consisting of a photosynthetic stem and foliage-like structures. Male and female gametangia develop at the tip of separate gametophores. The antheridia (male organs) produce many sperm, whereas the archegonia (the female organs) each form a single egg at the base (venter) of a flask-shaped structure. The archegonium produces attractant substances and at fertilization, the sperm swims down the neck to the venter and unites with the egg inside the archegonium. The zygote, protected by the archegonium, divides and grows into a sporophyte, still attached by its foot to the gametophyte.


Which of the following statements about the moss life cycle is false?

  1. The mature gametophyte is haploid.
  2. The sporophyte produces haploid spores.
  3. The calyptra buds to form a mature gametophyte.
  4. The zygote is housed in the venter.

The moss sporophyte is dependent on the gametophyte for nutrients. The slender seta (plural, setae), as seen in (Figure), contains tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the sporangium or capsule .


Spore mother cells in the sporangium undergo meiosis to produce haploid spores. The sporophyte has several features that protect the developing spores and aid in their dispersal. The calyptra, derived from the walls of the archegonium, covers the sporangium. A structure called the operculum is at the tip of the spore capsule. The calyptra and operculum fall off when the spores are ready for dispersal. The peristome, tissue around the mouth of the capsule, is made of triangular, close-fitting units like little “teeth.” The peristome opens and closes, depending on moisture levels, and periodically releases spores.

Section Summary

Seedless non-vascular plants are small, having the gametophyte as the dominant stage of the lifecycle. Without a vascular system and roots, they absorb water and nutrients on all their exposed surfaces. Collectively known as bryophytes, the three main groups include the liverworts, the hornworts, and the mosses. Liverworts are the most primitive plants and are closely related to the first land plants. Hornworts developed stomata and possess a single chloroplast per cell. Mosses have simple conductive cells and are attached to the substrate by rhizoids. They colonize harsh habitats and can regain moisture after drying out. The moss sporangium is a complex structure that allows release of spores away from the parent plant.

Visual Connection Questions

(Figure) Which of the following statements about the moss life cycle is false?


Basidiomycota: The Club Fungi

Figure 6. The fruiting bodies of a basidiomycete form a ring in a meadow, commonly called “fairy ring.” (Credit: “Cropcircles”/Wikipedia Commons)]

The fungi in the Phylum Basidiomycota are easily recognizable under a light microscope by their club-shaped fruiting bodies called basidia (singular, basidium), which are the swollen terminal cell of a hypha. The basidia, which are the reproductive organs of these fungi, are often contained within the familiar mushroom, commonly seen in fields after rain, on the supermarket shelves, and growing on your lawn. The fruiting bodies of a basidiomycete form a ring in a meadow, commonly called “fairy ring” (Figure 6). The best-known fairy ring fungus has the scientific name Marasmius oreades. The body of this fungus, its mycelium, is underground and grows outward in a circle. As it grows, the mycelium depletes the soil of nitrogen, causing the mycelia to grow away from the center and leading to the “fairy ring” of fruiting bodies where there is adequate soil nitrogen.

These mushroom-producing basidiomyces are sometimes referred to as “gill fungi” because of the presence of gill-like structures on the underside of the cap. The “gills” are actually compacted hyphae on which the basidia are borne. This group also includes shelf fungus, which cling to the bark of trees like small shelves. In addition, the basidiomycota includes smuts and rusts, which are important plant pathogens, and toadstools. Most edible fungi belong to the Phylum Basidiomycota however, some basidiomycetes produce deadly toxins. For example, Cryptococcus neoformans causes severe respiratory illness.

The lifecycle of basidiomycetes includes alternation of generations (Figure 7). Spores are generally produced through sexual reproduction, rather than asexual reproduction. The club-shaped basidium carries spores called basidiospores. In the basidium, nuclei of two different mating strains fuse (karyogamy), giving rise to a diploid zygote that then undergoes meiosis. The haploid nuclei migrate into basidiospores, which germinate and generate monokaryotic hyphae. The mycelium that results is called a primary mycelium. Mycelia of different mating strains can combine and produce a secondary mycelium that contains haploid nuclei of two different mating strains. This is the dikaryotic stage of the basidiomyces lifecyle and and it is the dominant stage. Eventually, the secondary mycelium generates a basidiocarp, which is a fruiting body that protrudes from the ground—this is what we think of as a mushroom. The basidiocarp bears the developing basidia on the gills under its cap.

Figure 7. The lifecycle of a basidiomycete alternates generation with a prolonged stage in which two nuclei (dikaryon) are present in the hyphae.

Practice Question

Which of the following statements is true?

  1. A basidium is the fruiting body of a mushroom-producing fungus, and it forms four basidiocarps.
  2. The result of the plasmogamy step is four basidiospores.
  3. Karyogamy results directly in the formation of mycelia.
  4. A basidiocarp is the fruiting body of a mushroom-producing fungus.

Filicinophyta

Filicinophyta (Pterophyta) A phylum of mainly terrestrial vascular plants (see tracheophyte) – the ferns. Ferns are perennial plants bearing large conspicuous leaves (fronds: see megaphyll) usually arising from either a rhizome or a short erect stem. Bracken is a common example. Only the tree ferns have stems that reach an appreciable height. There is a characteristic uncurling of the young leaves as they expand into the adult form. Reproduction is by means of spores borne on the underside of specialized leaves (sporophylls).

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Genes related to sporophyte development

While hornworts have a gametophyte-dominant life cycle like other bryophytes, their sporophyte generation (Fig. 3b) shows several unique features 25 . First, after fertilization, the zygote division in hornworts is longitudinal, whereas zygotes in all other land plants undergo transverse division. Second, the hornwort sporophyte maintains a basal sporophytic meristem producing cells that continuously differentiate into mature tissues towards the tip. A common origin of indeterminate sporophyte development in hornworts and vascular plant shoot apical meristem (SAM) has been hypothesized 25 . Lastly, hornwort sporophytes have stomata (Fig. 3c) similar to mosses and vascular plants, and the basic regulation may be shared across all stomatous lineages of land plants 26 . Nevertheless, firm evidence supporting the homology of meristems as well as stomata is scarce. Here, we found that multiple genes critical for flowering plant SAM and stomata function have homologues in the hornwort genomes and are preferentially expressed in the sporophyte phase.

Class 1 Knotted1-like homeobox (KNOX1) genes regulate sporophytic meristem activity in both P. patens and vascular plants 27 , while Class 2 Knotted1-like homeobox (KNOX2) genes maintain sporophyte cell fate in P. patens 28 . Interestingly, the KNOX1 orthologue is lost in the Anthoceros genomes and only KNOX2 genes were found (Supplementary Fig. 8 and Supplementary Tables 8 and 9). The KNOX2 orthologues showed strong sporophyte-specific expression (Fig. 3d), which implies that the involvement of KNOX2 in maintaining sporophytic cell fate may be conserved in all land plants. Heterodimerization of KNOX1/KNOX2 and BELL-LIKE HOMEOBOX proteins is a deeply conserved molecular mechanism that is required for the KNOX functions 29 . We found that in A. agrestis Bonn, a single BELL and a single KNOX2 gene were specifically expressed in the sporophyte phase. Nevertheless, contrary to our expectations, the BELL gene was more strongly expressed in the early stages while the KNOX2 gene in the later stages of sporophyte development (Fig. 3d and Supplementary Tables 8 and 9). This suggests that hornwort sporophyte identity may not be determined by KNOX2 through interaction with BELL. Nevertheless, this hypothesis needs functional verification because partially overlapping expression of the KNOX2 and BELL genes does not exclude the possibility of heterodimerization.

WUSCHEL-related homeobox 13 like (WOX13L) genes are involved in zygote development and stem cell formation in the moss P. patens 30 . A. thaliana WOX13 promotes replum formation in the fruit 31 and WOX14 promotes vascular cell differentiation 32 . The Anthoceros genomes have four WOX13L members (Supplementary Fig. 8 and Supplementary Tables 8 and 9) and WOX13La is specifically expressed in sporophytes while WOX13Lbcd have expression at both gametophyte and sporophyte generations (Fig. 3d) and may have diverse roles in stem cell maintenance and sporophyte development. The Anthoceros genomes also have a single FLORICAULA/LEAFY (FLO/LFY) gene (Supplementary Fig. 8 and Supplementary Tables 8 and 9), which in P. patens and A. thaliana controls zygote development and SAM maintenance, respectively 33 . In hornworts, LFY is predominantly expressed in the gametophyte stages (Fig. 3d) while in P. patens it is expressed both in the gametophyte and the sporophyte. It is possible that such differences may contribute to the unique developmental pattern of hornwort sporophytes.

Stomatal development in A. thaliana and P. patens is regulated by a conserved genetic toolbox, including the basic helix–loop–helix (bHLH) transcription factors SMF (SPCH, MUTE and FAMA), ICE/SCREAMs (SCRMs), EPIDERMAL PATTERNING FACTOR (EPF), ERECTA and TOO MANY MOUTHS (TMM) genes 34,35 . FAMA in particular is involved in the final guard cell differentiation step and serves as the key switch. Orthologues of SMF, TMM and EPF were absent in M. polymorpha, consistent with the fact that liverworts do not have stomata 8 . We found orthologues of FAMA (SMF), SCRM, ERECTA, EPF and TMM in the Anthoceros genomes (in line with a previous study based on our earlier genome draft 26 Supplementary Table 10 and Supplementary Fig. 9). SMF, SCRM, TMM and EPF showed sporophyte-specific expression patterns (Fig. 3d), suggesting that they may have similar roles in stomatal patterning in hornworts. While ERECTA was also expressed during early sporophyte development, its expression fluctuated between replicates and results were inconclusive. EPF expression showed similar inconsistency among replicates but did not influence our conclusion about its sporophyte-specific expression. In addition to EPF, an EPF-like gene in the EPFL4-6 clade, was found in hornworts (Supplementary Fig. 9), and is specifically expressed in gametophytes with a higher expression toward maturity and thus perhaps involved in a different cell–cell signalling other than stomatal regulation. EPF4 and EPF6 in A. thaliana are involved in coordination of the central and peripheral zone in SAM 36 . Taken together, our data are consistent with a single origin of stomatal differentiation mechanism among all stomatous land plants, though positional determination may have evolved differently (Supplementary Notes).


Contents

Major groups in large type.

Ecdysozoa Edit

  • Nematoda: the round worms. For purists, the name Nemata has priority. [4] Despite their rather limited body form, this is a major phylum, with huge numbers in every conceivable habitat. "More than 15,000 species have been described, of an estimated one million living species". [5] p90 Nematodes include both free-living and parasitic species of plants and animals, including man. Of their large number of species most are likely to be parasites. [6] Nematodes are one of the few life-forms in which each species has a defined number of cells. [7] : small group of nematode-like parasites. They spend their larval stage in the body cavity of arthropods. The adult stage is free, but non-feeding, though it may live for several months. About 250+ species. [5] p85 or Priapula: small phylum of 18 species, with large front section which can be drawn back into the body cavity and extruded for feeding. The larger species are carnivores, seizing prey. The Burgess Shalefauna from the Cambrian shows that the living species are but a remnant of a once much larger group. [2] p358 : another small phylum with an introvert that carries a mouth at the end when extended. [5] p97 Two groups, described as classes in Sørensen. [8] 270 species have been described and many more are expected. [9][10] : a new phylum, discovered in the 1970s. They are microscopic, 100–485μm [11]

Lophotrochozoa Edit

    : 150 species, no certain fossil record. Small, tube-like marine animals with long tentacle-like front part which can be pulled in or out. The mouth is surrounded by a ring of cilia. Has pelagic larvae.
  • Mollusca: a great phylum by number of species and by variety of body forms largely aquatic. Hugely important fossil record from the Lower Cambrian. A major food source for mankind, second only to fish. United by their mantle, the muscular 'foot', the radula (teeth band), and (ancestrally) by the shell. Number of living species estimated as 50,000 to 150,000. Classes: lesser classes are the Aplacophora, Monoplacophora, and Polyplacophora. Major classes are the Gastropods, Cephalopods, Bivalves and Scaphopods. A familiarity with bivalve evolution is valuable for identifying strata, so common are their fossils. Larvae are trochophores or veligers (many gastropods & bivalves) glochidium (some freshwater bivalves).
  • Annelida: important phylum of both aquatic and terrestrial segmented worms. At least 15,000 living species. Fossil record weak, evolutionary history not well known. Classes: Polychaeta (marine worms), Oligochaeta (earthworms), Hirudinea (leeches). Larvae are trochophores or nectochaeta.
  • Bryozoa, also known as the Ectoprocta: An aquatic phylum with a huge fossil record (one of the most common in the Palaeozoic). Still fairly common, though little known to the public. There are now 5000 species, most of which build calcareous skeletons. They are almost all colonial, and all their zooids are clones. : A very small phylum, with 12 species. Live on the sea floor (benthic), build chitinous tubes covered with mud or sand or bore into calcareous rock. Usually have horeshoe-shaped lophophores with ciliated tentacles. or Nemertini: flat, unsegmented ribbon worms, mostly aquatic. They have also been called Rhynchocoela or proboscis worms. About 1400 species. There have been reports of extremely long ribbon worms, unconfirmed. Larvae are pilidiums.
  • Platyhelminthes: the flatworms. Classes: Turbellaria: free-living and aquatic (4,500 species) Trematoda: parasitic flukes of molluscs and vertebrates (

Deuterostomia Edit

    : A group of marine benthic worms consisting of 3 main lineages defined by a blind gut, a net-like nervous system, and lack of nephridia. The position of this group on the tree of life is currently debated as either the sister group to all other Bilateria [18] or as sister to all other Deuterostomia[19]
      : Benthic marine worms, often found in the deeps sea, sizes between 2–20cm. [20] : very small marine worms (usually under 2 millimeters in length) found in marine and brackish waters usually living in the benthos. : small marine worms.
    • Echinodermata: One of the most important marine phyla, with radial symmetry. 17,000 living species, which all live in the ocean, mostly on the sea bed. This is the largest phylum which is entirely marine. The main classes are quite well-known. The Crinoids are 'sea lilies', a remnant of a once great clade the Asterozoa are the starfish, major predators of shell-fish, and the brittle stars. The Echinozoa are the sea urchins, sand dollars and the sea cucumbers. There are also some extinct groups. The echinoderm fossil record is extensive. Larvae are varied and planktonic: pluteus (echinoids) dipleurula, then bipinneria then brachiolaria (starfish) ophiopluteus (brittle stars) doliolaris (sea cucumbers). : The Chordates' closest relatives, three groups which are brought together in most modern taxonomies.
        : the acorn worms. A small, well-defined group with 70 marine species. Relatives of the chordates.
    • †Graptolites: fossil colonial animals. : a small sub-phylum of two or three marine groups which usually build tubes, and form small colonies on sea floor. They have a long fossil record. Zooids carry prominent ciliated tentacles.
      • : the tunicates. : the lancelates, such as the former Amphioxus. or Vertebrata: the vertebrates. About 60,000 species recognised. The term vertebrate usually now excludes the lamprey and hagfish, which are included in the broader term craniate.

      Other Bilateria phyla Edit

        : a recently discovered group of tiny animals which live on lobsters. One genus and three species so far. : jaw worms, a small phylum of small marine animals (100 species). Hermaphrodite, live in muddy benthic habitat, scape food from particles with their jaw. : arrow worms. Only about 120 species, but huge numbers in the plankton some are benthic. They are predators, up to 12 cm long. They use a neurotoxin to subdue prey. : a small phylum of parasites of marine invertebrates.

      Non-Bilateria Edit

        : a large phylum, with an extensive fossil record. 10,000 living species. Aquatic, mainly marine, five classes:
          (sea anemones, corals) (true jellyfish) (box jellies) (stalked jellyfish). : the hydroids
          : Two described species: Trichoplax adhaerens, discovered in 1883 and ```Hoilungia hongkongensis```. [22] Small, about 2mm, aquatic, eats bacteria and single-celled algae & protozoa.
          : sponges. 5000 species, aquatic mainly marine but several fresh water species, Have collared cells with long cilia. Sessile, have cell differentiation. [23] Skeleton are of spongin, or are calcareous CaCO3, or silicious SiO2.

        At least 21 phyla are exclusively aquatic, with several others in quasi-aquatic habitats on land. None are entirely terrestrial. This is testimony to the importance of water for life, and to the sea in particular. It is fairly certain that all phyla originated in the sea or, at any rate, in water. Most made their first showing in the Cambrian, or in the Ediacaran. Most of the soft-bodied phyla have left few fossils.

        Phyla may be grouped according to evidence about their evolutionary relationships. The list above puts similar groups together.

        This kind of megataxonomy is becoming more convincing as DNA sequence analysis proceeds through the phyla. Some entirely fossil groups are still placed where they are on anatomy and commonsense rather than hard molecular evidence. The trilobites are a good example. Their position in the Arthropoda is based on not much more than their bilateral symmetry and an exoskeleton. These groupings are discussed further in the references to this page. [1] [2] [16]

        This table has the advantage of being sortable. The terminology differs in places from the above descriptions. Also, by listing living species only for most phyla, those with huge fossil records (like Bryozoa and Brachiopods) are lower in the order despite being important aquatic forms in the Palaeozoic era.

        Phylum Meaning Common name Distinguishing characteristic Species described
        Acanthocephala Thorny headed worms Thorny-headed worms Reversible spiny proboscis. Now usually included in Rotifera. 7003132900000000000♠ approx. 1,329 extant (= living)
        Acoelomorpha Without gut Acoels No mouth or alimentary canal (alimentary canal = digestive tract in digestive system) 483
        Annelida Little ring Segmented worms Multiple circular segment 20,481+ extant
        Arthropoda Jointed foot Arthropods Chitin exoskeleton 7006110673800000000♠ 1,106,738+
        Brachiopoda Arm foot Lamp shells Lophophore and pedicle 11,082 extant
        Bryozoa Moss animals Moss animals, sea mats Lophophore, no pedicle, ciliated tentacles 5,609 extant
        Chaetognatha Longhair jaw Arrow worms Chitinous spines either side of head, fins 132 extant
        Chordata With a cord Chordates Hollow dorsal nerve cord, notochord, pharyngeal slits, endostyle, post-anal tail 65,000+
        Cnidaria Stinging nettle Coelenterates Nematocysts (stinging cells) 11,791
        Ctenophora Comb bearer Comb jellies Eight "comb rows" of fused cilia 210 extant
        Cycliophora Wheel carrying Symbion Circular mouth surrounded by small cilia 2
        Echinodermata Spiny skin Echinoderms Fivefold radial symmetry in living forms, mesodermal calcified spines 10,832
        Entoprocta Inside anus Goblet worm Anus inside ring of cilia 171
        Gastrotricha Hair stomach Meiofauna Two terminal adhesive tubes 851
        Gnathostomulida Jaw orifice Jaw worms 101
        Hemichordata Half cord Acorn worms, pterobranchs Stomochord in collar, pharyngeal slits 139
        Kinorhyncha Motion snout Mud dragons Eleven segments, each with a dorsal plate 188
        Loricifera Corset bearer Brush heads Umbrella-like scales at each end 27
        Micrognathozoa Tiny jaw animals Accordion-like extensible thorax. Newly discovered close to Rotifers. 7000100000000000000♠ 1
        Mollusca Soft Mollusks / molluscs Muscular foot and mantle round shell 85,844
        Nematoda Thread like Round worms Round cross section, keratin cuticle 3,452
        Nematomorpha Thread form Horsehair worms 361
        Nemertea A sea nymph Ribbon worms 1,351
        Onychophora Claw bearer Velvet worms Legs tipped by chitinous claws 205
        Orthonectida Straight swim Single layer of ciliated cells surrounding a mass of sex cells 25
        Phoronida Zeus's mistress Horseshoe worms U-shaped gut 19
        Placozoa Plate animals 2
        Platyhelminthes Flat worms Flat worms 18,089
        Porifera* Pore bearer Sponges Perforated interior wall 9,049
        Priapulida Little Priapus 22
        Rhombozoa Lozenge animal Single axial cell form front to back, surrounded by ciliated cells 7001750000000000000♠ 75
        Rotifera Wheel bearer Rotifers crown of cilia at front 2,011
        Sipuncula Small tube Peanut worms Mouth surrounded by invertible tentacles 205
        Tardigrada Slow step Water bears Four segmented body and head 1,018
        Xenoturbellida Strange flatworm Ciliated deuterostome 4
        Total: 35 1,356,899 and more species being discovered every day
        Protostome Bilateria
        Deuterostome
        Basal/disputed
        Others (Radiata or Parazoa)

        This list is to help when you read older literature which may use out-of-date terms.


        Watch the video: The Reproductive Lives of Nonvascular Plants: Alternation of Generations - Crash Course Biology #36 (May 2022).


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