Information

26.1A: The Evolution of Seed Plants and Adaptations for Land - Biology

26.1A: The Evolution of Seed Plants and Adaptations for Land - Biology


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

The evolution of seeds allowed plants to reproduce independently of water; pollen allows them to disperse their gametes great distances.

Learning Objectives

  • Recognize the significance of seed plant evolution

Key Points

  • Plants are used for food, textiles, medicines, building materials, and many other products that are important to humans.
  • The evolution of seeds allowed plants to decrease their dependency upon water for reproduction.
  • Seeds contain an embryo that can remain dormant until conditions are favorable when it grows into a diploid sporophyte.
  • Seeds are transported by the wind, water, or by animals to encourage reproduction and reduce competition with the parent plant.

Key Terms

  • seed: a fertilized ovule, containing an embryonic plant
  • sporophyte: a plant (or the diploid phase in its life cycle) that produces spores by meiosis in order to produce gametophytes
  • pollen: microspores produced in the anthers of flowering plants

Evolution of Seed Plants

The lush palms on tropical shorelines do not depend upon water for the dispersal of their pollen, fertilization, or the survival of the zygote, unlike mosses, liverworts, and ferns of the terrain. Seed plants, such as palms, have broken free from the need to rely on water for their reproductive needs. They play an integral role in all aspects of life on the planet, shaping the physical terrain, influencing the climate, and maintaining life as we know it. For millennia, human societies have depended upon seed plants for nutrition and medicinal compounds; and more recently, for industrial by-products, such as timber and paper, dyes, and textiles. Palms provide materials including rattans, oils, and dates. Wheat is grown to feed both human and animal populations. The fruit of the cotton boll flower is harvested as a boll, with its fibers transformed into clothing or pulp for paper. The showy opium poppy is valued both as an ornamental flower and as a source of potent opiate compounds.

Seeds and Pollen as an Evolutionary Adaptation to Dry Land

Unlike bryophyte and fern spores (which are haploid cells dependent on moisture for rapid development of gametophytes ), seeds contain a diploid embryo that will germinate into a sporophyte. Storage tissue to sustain growth and a protective coat give seeds their superior evolutionary advantage. Several layers of hardened tissue prevent desiccation, freeing reproduction from the need for a constant supply of water. Furthermore, seeds remain in a state of dormancy induced by desiccation and the hormone abscisic acid until conditions for growth become favorable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition with the parent plant.

Pollen grains are male gametophytes carried by wind, water, or a pollinator. The whole structure is protected from desiccation and can reach the female organs without dependence on water. Male gametes reach female gametophyte and the egg cell gamete though a pollen tube: an extension of a cell within the pollen grain. The sperm of modern gymnosperms lack flagella, but in cycads and the Gingko, the sperm still possess flagella that allow them to swim down the pollen tube to the female gamete; however, they are enclosed in a pollen grain.


26.1A: The Evolution of Seed Plants and Adaptations for Land - Biology

By the end of this section, you will have completed the following objectives:

  • Explain when seed plants first appeared and when gymnosperms became the dominant plant group
  • Describe the two major innovations that allowed seed plants to reproduce in the absence of water
  • Discuss the purpose of pollen grains and seeds
  • Describe the significance of angiosperms bearing both flowers and fruit

The first plants to colonize land were most likely closely related to modern day mosses (bryophytes) and are thought to have appeared about 500 million years ago. They were followed by liverworts (also bryophytes) and primitive vascular plants—the pterophytes—from which modern ferns are derived. The lifecycle of bryophytes and pterophytes is characterized by the alternation of generations, like gymnosperms and angiosperms what sets bryophytes and pterophytes apart from gymnosperms and angiosperms is their reproductive requirement for water. The completion of the bryophyte and pterophyte life cycle requires water because the male gametophyte releases sperm, which must swim—propelled by their flagella—to reach and fertilize the female gamete or egg. After fertilization, the zygote matures and grows into a sporophyte, which in turn will form sporangia or “spore vessels.” In the sporangia, mother cells undergo meiosis and produce the haploid spores. Release of spores in a suitable environment will lead to germination and a new generation of gametophytes.

In seed plants, the evolutionary trend led to a dominant sporophyte generation, and at the same time, a systematic reduction in the size of the gametophyte: from a conspicuous structure to a microscopic cluster of cells enclosed in the tissues of the sporophyte. Whereas lower vascular plants, such as club mosses and ferns, are mostly homosporous (produce only one type of spore), all seed plants, or spermatophytes, are heterosporous. They form two types of spores: megaspores (female) and microspores (male). Megaspores develop into female gametophytes that produce eggs, and microspores mature into male gametophytes that generate sperm. Because the gametophytes mature within the spores, they are not free-living, as are the gametophytes of other seedless vascular plants. Heterosporous seedless plants are seen as the evolutionary forerunners of seed plants.

Seeds and pollen—two critical adaptations to drought, and to reproduction that doesn’t require water—distinguish seed plants from other (seedless) vascular plants. Both adaptations were required for the colonization of land begun by the bryophytes and their ancestors. Fossils place the earliest distinct seed plants at about 350 million years ago. The first reliable record of gymnosperms dates their appearance to the Pennsylvanian period, about 319 million years ago (Figure 1). Gymnosperms were preceded by progymnosperms, the first naked seed plants, which arose about 380 million years ago. Progymnosperms were a transitional group of plants that superficially resembled conifers (cone bearers) because they produced wood from the secondary growth of the vascular tissues however, they still reproduced like ferns, releasing spores into the environment. Gymnosperms dominated the landscape in the early (Triassic) and middle (Jurassic) Mesozoic era. Angiosperms surpassed gymnosperms by the middle of the Cretaceous (about 100 million years ago) in the late Mesozoic era, and today are the most abundant plant group in most terrestrial biomes.

Figure 1. Various plant species evolved in different eras. (credit: United States Geological Survey)

Pollen and seed were innovative structures that allowed seed plants to break their dependence on water for reproduction and development of the embryo, and to conquer dry land. The pollen grains are the male gametophytes, which contain the sperm (gametes) of the plant. The small haploid (1n) cells are encased in a protective coat that prevents desiccation (drying out) and mechanical damage. Pollen grains can travel far from their original sporophyte, spreading the plant’s genes. The seed offers the embryo protection, nourishment, and a mechanism to maintain dormancy for tens or even thousands of years, ensuring germination can occur when growth conditions are optimal. Seeds therefore allow plants to disperse the next generation through both space and time. With such evolutionary advantages, seed plants have become the most successful and familiar group of plants, in part because of their size and striking appearance.


Gymnosperms

Gymnosperms (“naked seed”) are a diverse group of seed plants and are paraphyletic. Paraphyletic groups do not include descendants of a single common ancestor. Gymnosperm characteristics include naked seeds, separate female and male gametes, pollination by wind, and tracheids, which transport water and solutes in the vascular system.

Life Cycle of a Conifer

Pine trees are conifers and carry both male and female sporophylls on the same plant. Like all gymnosperms, pines are heterosporous and produce male microspores and female megaspores. In the male cones, or staminate cones, the microsporocytes give rise to microspores by meiosis. The microspores then develop into pollen grains. Each pollen grain contains two cells: one generative cell that will divide into two sperm, and a second cell that will become the pollen tube cell. In the spring, pine trees release large amounts of yellow pollen, which is carried by the wind. Some gametophytes will land on a female cone. The pollen tube grows from the pollen grain slowly, and the generative cell in the pollen grain divides into two sperm cells by mitosis. One of the sperm cells will finally unite its haploid nucleus with the haploid nucleus of an egg cell in the process of fertilization.

Female cones , or ovulate cones, contain two ovules per scale. One megasporocyte undergoes meiosis in each ovule. Only a single surviving haploid cell will develop into a female multicellular gametophyte that encloses an egg. On fertilization, the zygote will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant. Fertilization and seed development is a long process in pine trees—it may take up to two years after pollination. The seed that is formed contains three generations of tissues: the seed coat that originates from the parent plant tissue, the female gametophyte that will provide nutrients, and the embryo itself. Figure 1 illustrates the life cycle of a conifer.


Evolution of Angiosperms

Undisputed fossil records place the massive appearance and diversification of angiosperms in the middle to late Mesozoic era. Angiosperms (“seed in a vessel”) produce a flower containing male and/or female reproductive structures. Fossil evidence (Figure) indicates that flowering plants first appeared in the Lower Cretaceous, about 125 million years ago, and were rapidly diversifying by the Middle Cretaceous, about 100 million years ago. Earlier traces of angiosperms are scarce. Fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm leaves. By the mid-Cretaceous, a staggering number of diverse flowering plants crowd the fossil record. The same geological period is also marked by the appearance of many modern groups of insects, including pollinating insects that played a key role in ecology and the evolution of flowering plants.

Although several hypotheses have been offered to explain this sudden profusion and variety of flowering plants, none have garnered the consensus of paleobotanists (scientists who study ancient plants). New data in comparative genomics and paleobotany have, however, shed some light on the evolution of angiosperms. Rather than being derived from gymnosperms, angiosperms form a sister clade (a species and its descendents) that developed in parallel with the gymnosperms. The two innovative structures of flowers and fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic variability and range. Paleobotanists debate whether angiosperms evolved from small woody bushes, or were basal angiosperms related to tropical grasses. Both views draw support from cladistics studies, and the so-called woody magnoliid hypothesis—which proposes that the early ancestors of angiosperms were shrubs—also offers molecular biological evidence.

The most primitive living angiosperm is considered to be Amborella trichopoda, a small plant native to the rainforest of New Caledonia, an island in the South Pacific. Analysis of the genome of A. trichopoda has shown that it is related to all existing flowering plants and belongs to the oldest confirmed branch of the angiosperm family tree. A few other angiosperm groups called basal angiosperms, are viewed as primitive because they branched off early from the phylogenetic tree. Most modern angiosperms are classified as either monocots or eudicots, based on the structure of their leaves and embryos. Basal angiosperms, such as water lilies, are considered more primitive because they share morphological traits with both monocots and eudicots.

This leaf imprint shows a Ficus speciosissima, an angiosperm that flourished during the Cretaceous period. (credit: W. T. Lee, USGS)


The Evolution of Seed Plants

Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems.

A seed consists of an embryo and nutrients surrounded by a protective coat.

The gametophytes of seed plants develop within the walls of spores that are retained within tissues of the parent sporophyte.

What human reproductive organ is functionally similar to this seed?

Seeds and pollen grains are key adaptations for life on land

In addition to seeds, the following are common to all seed plants:

Gametophyte / sporophyte relationships in different plant groups

Reduced (usually microscopic), dependent on surrounding sporophyte tissue for nutrition

Reduced, independent (photosynthetic and free-living)

Reduced, dependent on gametophyte for nutrition

Mosses and other nonvascular plants

Ferns and other seedless vascular plants

Seed plants (gymnosperms and angiosperms)

Microscopic female gametophytes (n) inside ovulate cone

Microscopic male gametophytes (n) inside pollen cone

Microscopic female gametophytes (n) inside these parts of flowers

Microscopic male gametophytes (n) inside these parts of flowers


Seed Habit: Meaning, Origin and Evolution

Seed is a ripened or fertilised ovule. A seed (ovule) may be defined as an integumented indehiscent megasporangium. There is further elabora­tion of the megasporangium so as to enable it to be an ideal starting point for the development of a new plant.

Thus seeds provide parental care to the land plants, where megagametophyte is retained within the indehiscent megasporangium and is well-protected by the integument. Moreover, apex of the nucellus is elaborated for reception of microspore and fertilisation takes place through pollen tube or pollen tube-like structure.

Origin of Seed Habit:

Stewart and Rothwell (1993) proposed six evolutionary events that took place in ovule evo­lution after the onset of heterospory:

1. Degeneration of three megaspores and formation of a single functional megas­pore in the megasporangium.

2. Retention of the functional megaspore in the megasporangium (nucellus) until embryo development.

3. Formation of endosporic megagametophytes within an indehiscent megaspo­rangium.

4. Elaboration of the apical part of nucellus to receive microspores or pollen grains.

5. Formation of an integument which delimited a micropyle.

6. Formation of pollen tube or pollen tube-­like structure from endosporic microgametophyte.

Due to the incomplete records of fossil plants it is difficult to predict the exact order of these six events. However, the evolution of seed habit can be evaluated on the basis of above- mentioned events.

Evolution of Integument in Seed Habit:

Several theories have been put forward about the origin of integument.

According to the synangial hypothesis by Benson (1904), the integument evolved from the sterilisation of the outer ring of sporangia in a radial synangium.

The nucellar modification theory was pro­posed by Andrews (1961) based on the structure of megasporangium of Stauropteris burntislandica, a coenopterid fern. According to this theory, a reduction of megaspore from two to one took place in one which was sunk towards the base of the sporangium (Fig. 7.144).

The simple seed was evolved by further proliferation of the spo­rangial wall and the subsequent division of the basal vascular bundle to extend into the newly formed integument. The occurrence of Palaeozoic seeds such as Lagenostoma and Conostoma supports this hypothesis.

The most acceptable hypothesis is the telome concept by Zimmermann in 1952. According to this hypothesis, a dichotomously branched axial system bearing terminal sporan­gia was the starting point (Fig. 7.145A).

There was a gradual reduction of some of the axis and one of the sporangia becomes surrounded by an aggregation of sterile telomes (Fig. 7.145B, C). The fusion of the telomes resulted in the forma­tion of the integument (Fig. 7.145D). Thus the primitive seed (preovule) consisted of a naked megasporangium surrounded by a ring of vascularised integumentary lobes.

The evidences in support of the origin of seed (ovule) following telome hypothesis came into existence after the discovery of several ovule-like structures from the Upper Devonian and Lower Carboniferous strata. Though these ovule-like structures fulfilled most of the seed characteristics, they lack a well-defined micropyle.

Stewart and Rothwell (1993) proposed the term ‘pre-ovule’ for such structures. A pre-ovule may be defined as an ovule-like structure con­sisting of a megasporangium which is either naked or invested by unfused or partially fused integumentary lobes and thus lacks a well- defined micropyle.

The investigations of Upper Devonian and Lower Carboniferous pre-ovules have provided important clues in documenting the transition between pre-ovules and the true ovules (seeds).

Archaeosperma arnoldii, a pre-ovule bearing organ reported by Pettitt and Beck (1968) from Upper Devonian, consists of a cupule that par­tially surrounds four pre-ovules (Fig. 7.146). The cupules are planted dichotomously branched axes consisting of sterile telome trusses that are webbed in the proximal part.

Each cupule con­tains two short pedicels, each bearing a small orthotropous pre-ovule. The integument is serrat­ed at the apex of each pre-ovule into a number of lobes that form a rudimentary micropyle.

Several other pre-ovules have been described from Upper Devonian. Elkinsia and Moresnetia are two other important pre-ovules.

Elkinsia consists of loose tufts of cupules produced on a cruciately forked branching sys­tem. Cupules are produced either singly or in pairs. Each cupule consists of 16 sterile branch tips surrounding a total of four orthotropous ovules. The integument is made up of four or five lobes that are fused at the basal region only and thus lacks a well-defined micropyle.

Moresnetia produced cupules much like those of Elkinsia. Moresnetia consists of 8 to 10 thin integumentary lobes that are only fused at the base and tends to flare away from the pre- ovule apex.

Evolution of ovules from pre-ovules:

The evidence in support of the fusion of telomes (integumentary lobes) of pre-ovules to form integument with its micropyle is well-docu­mented by A. G. Long (1966). Long described a number of Lower Carboniferous pre-ovules from Scotland.

In Genomosperma kidstoni, the integu­ment is represented by a ring of eight unfused telomes surrounding a centrally placed megaspo­rangium (fig. 7.147A). Further fusion took place in Genomosperma latenswhere eight syntelomes are fused from the base distally for about one- third of their length (Fig. 7.147B).

In Physostoma elegans, the syntelomes are fused for one-half of their length. The more advanced degree of fusion is noted in Archaeosperma arnoldii and Eurystoma angulare where four syntelomes are fused from the base distally for about three-fourth of their length and still lacks a well-defined micropyle (Fig. 7.147).

Stamnostoma huttonense has reached the level of an ovule where the fusion of syntelomes is complete that delimited a micropyle (Fig. 7.147D).

Thus, the Lower Carboniferous ovules (seeds) demonstrate the direct relationship between the degree of fusion of integumen­tary lobes and concomitant formation of micropyle.


Evolution of Angiosperms

The roughly 200 million years between the appearance of the gymnosperms and the flowering plants gives us some appreciation for the evolutionary experimentation that ultimately produced flowers and fruit. Angiosperms (“seed in a vessel”) produce a flower containing male and/or female reproductive structures. Fossil evidence ((Figure)) indicates that flowering plants first appeared about 125 million years ago in the Lower Cretaceous (late in the Mesozoic era), and were rapidly diversifying by about 100 million years ago in the Middle Cretaceous. Earlier traces of angiosperms are scarce. Fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm leaves. By the mid-Cretaceous, a staggering number of diverse flowering plants crowd the fossil record. The same geological period is also marked by the appearance of many modern groups of insects, suggesting that pollinating insects played a key role in the evolution of flowering plants.

New data in comparative genomics and paleobotany (the study of ancient plants) have shed some light on the evolution of angiosperms. Although the angiosperms appeared after the gymnosperms, they are probably not derived from gymnosperm ancestors. Instead, the angiosperms form a sister clade (a species and its descendents) that developed in parallel with the gymnosperms. The two innovative structures of flowers and fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic variability and range. There is no current consensus on the origin of the angiosperms. Paleobotanists debate whether angiosperms evolved from small woody bushes, or were related to the ancestors of tropical grasses. Both views draw support from cladistics, and the so-called woody magnoliid hypothesis—which proposes that the early ancestors of angiosperms were shrubs like modern magnolia—also offers molecular biological evidence.

The most primitive living angiosperm is considered to be Amborella trichopoda, a small plant native to the rainforest of New Caledonia, an island in the South Pacific. Analysis of the genome of A. trichopoda has shown that it is related to all existing flowering plants and belongs to the oldest confirmed branch of the angiosperm family tree. The nuclear genome shows evidence of an ancient whole-genome duplication. The mitochondrial genome is large and multichromosomal, containing elements from the mitochondrial genomes of several other species, including algae and a moss. A few other angiosperm groups, called basal angiosperms, are viewed as having ancestral traits because they branched off early from the phylogenetic tree. Most modern angiosperms are classified as either monocots or eudicots, based on the structure of their leaves and embryos. Basal angiosperms, such as water lilies, are considered more ancestral in nature because they share morphological traits with both monocots and eudicots.


Flowers and Fruits as an Evolutionary Adaptation

Angiosperms produce their gametes in separate organs, which are usually housed in a flower. Both fertilization and embryo development take place inside an anatomical structure that provides a stable system of sexual reproduction largely sheltered from environmental fluctuations. With about 300,000 species, flowering plants are the most diverse phylum on Earth after insects, which number about 1,200,000 species. Flowers come in a bewildering array of sizes, shapes, colors, smells, and arrangements. Most flowers have a mutualistic pollinator, with the distinctive features of flowers reflecting the nature of the pollination agent. The relationship between pollinator and flower characteristics is one of the great examples of coevolution.

Following fertilization of the egg, the ovule grows into a seed. The surrounding tissues of the ovary thicken, developing into a fruit that will protect the seed and often ensure its dispersal over a wide geographic range. Not all fruits develop completely from an ovary such “false fruits” or pseudocarps, develop from tissues adjacent to the ovary. Like flowers, fruit can vary tremendously in appearance, size, smell, and taste. Tomatoes, green peppers, corn, and avocados are all examples of fruits. Along with pollen and seeds, fruits also act as agents of dispersal. Some may be carried away by the wind. Many attract animals that will eat the fruit and pass the seeds through their digestive systems, then deposit the seeds in another location. Cockleburs are covered with stiff, hooked spines that can hook into fur (or clothing) and hitch a ride on an animal for long distances. The cockleburs that clung to the velvet trousers of an enterprising Swiss hiker, George de Mestral, inspired his invention of the loop and hook fastener he named Velcro.


Evolution of Seed Plants

The first plants to colonize land were most likely closely related to modern day mosses (bryophytes) and are thought to have appeared about 500 million years ago. They were followed by liverworts (also bryophytes) and primitive vascular plants—the pterophytes—from which modern ferns are derived. The lifecycle of bryophytes and pterophytes is characterized by the alternation of generations, like gymnosperms and angiosperms what sets bryophytes and pterophytes apart from gymnosperms and angiosperms is their reproductive requirement for water.

The completion of the bryophyte and pterophyte life cycle requires water because the male gametophyte releases sperm, which must swim—propelled by their flagella—to reach and fertilize the female gamete or egg. After fertilization, the zygote matures and grows into a sporophyte, which in turn will form sporangia or “spore vessels.” In the sporangia, mother cells undergo meiosis and produce the haploid spores. Release of spores in a suitable environment will lead to germination and a new generation of gametophytes.

In seed plants, the evolutionary trend led to a dominant sporophyte generation, and at the same time, a systematic reduction in the size of the gametophyte: from a conspicuous structure to a microscopic cluster of cells enclosed in the tissues of the sporophyte. Whereas lower vascular plants, such as club mosses and ferns, are mostly homosporous (produce only one type of spore), all seed plants, or spermatophytes, are heterosporous.

They form two types of spores: megaspores (female) and microspores (male). Megaspores develop into female gametophytes that produce eggs, and microspores mature into male gametophytes that generate sperm. Because the gametophytes mature within the spores, they are not free-living, as are the gametophytes of other seedless vascular plants. Heterosporous seedless plants are seen as the evolutionary forerunners of seed plants.

Seeds and pollen—two critical adaptations to drought, and to reproduction that doesn’t require water—distinguish seed plants from other (seedless) vascular plants. Both adaptations were required for the colonization of land begun by the bryophytes and their ancestors. Fossils place the earliest distinct seed plants at about 350 million years ago. The first reliable record of gymnosperms dates their appearance to the Pennsylvanian period, about 319 million years ago (see the figure below). Gymnosperms were preceded by progymnosperms, the first naked seed plants, which arose about 380 million years ago.

Progymnosperms were a transitional group of plants that superficially resembled conifers (cone bearers) because they produced wood from the secondary growth of the vascular tissues however, they still reproduced like ferns, releasing spores into the environment. Gymnosperms dominated the landscape in the early (Triassic) and middle (Jurassic) Mesozoic era. Angiosperms surpassed gymnosperms by the middle of the Cretaceous (about 100 million years ago) in the late Mesozoic era, and today are the most abundant plant group in most terrestrial biomes.

Various plant species evolved in different eras. (credit: United States Geological Survey)

Pollen and seed were innovative structures that allowed seed plants to break their dependence on water for reproduction and development of the embryo, and to conquer dry land. The pollen grains are the male gametophytes, which contain the sperm (gametes) of the plant. The small haploid (1n) cells are encased in a protective coat that prevents desiccation (drying out) and mechanical damage. Pollen grains can travel far from their original sporophyte, spreading the plant’s genes.

The seed offers the embryo protection, nourishment, and a mechanism to maintain dormancy for tens or even thousands of years, ensuring germination can occur when growth conditions are optimal. Seeds therefore allow plants to disperse the next generation through both space and time. With such evolutionary advantages, seed plants have become the most successful and familiar group of plants, in part because of their size and striking appearance.


Critical appraisal

In the classification above, only the major divisions and classes of living plants are listed, and a number of entirely extinct divisions are omitted. The classification outlined is somewhat conservative but is one that best conforms to available data and has gained wide acceptance.

Biological classifications were initially mechanical or “artificial” that is to say, they had no basis in evolution. This was followed by a period of “natural system” construction, whereby plants were grouped together on the basis of their overall similarities or differences, using as many characteristics as possible. Contemporary systems of biological classification are phylogenetic, which means that various plants are arranged together because they are thought to be related by descent from a common ancestor. As additional molecular evidence has become available, classifications have changed to accommodate the new information.

At the turn of the 19th century, the plant kingdom was frequently divided into two major groups, the cryptogamia (algae, fungi, bryophytes, and ferns) and the phanerogamia (gymnosperms and angiosperms). Subsequently, it was common practice among systematic botanists to group all vascular plants together under a single division, Tracheophyta. More modern taxonomies, such as those of the Angiosperm Phylogeny Group (APG), do not formally recognize groupings at the division level but use more informal groups known as clades, a view that interprets the individual major groups to be less closely related to one another than was previously believed. Difficult and complex questions still exist in the definition and circumscription of certain groups. The phylogenetic relationships, if any, of the bryophytic plants with primitive vascular plants remain unclear.


Watch the video: Population Ecology: The Texas Mosquito Mystery - Crash Course Ecology #2 (June 2022).


Comments:

  1. Dukus

    is there something similar?

  2. Karney

    Tell details ..

  3. Aethelhere

    Rather good idea

  4. Blaeey

    I think you will allow the mistake. I offer to discuss it.

  5. Rane

    Absolutely with you it agree. In it something is and it is excellent idea. It is ready to support you.



Write a message