Notes

VASCULAR PLANTS

The early plants and mosses developed close to water and on damp ground. These plants were rootless and depended on the process of osmosis between the cells to transport water. Osmosis is the diffusion of water across a membrane from a region of higher water content to one with lower water content. However, as plants evolved, they moved farther inland, away from water bodies. Here, the moisture content in the soil was lower. To obtain water for survival, plants developed roots that penetrated deep into the ground. The first plants with roots were club mosses, horsetails, and ferns.

These three plants had certain characteristics in common:

(1) presence of roots and leaves, (2) transport of water through vessels in the stems, and (3) rigidity of structure with height.

Club mosses, horsetails, and ferns were categorized as the first seedless vascular plants. Vascular plants are defined as plants that have specialized tissues, called vascular tissues, for conducting water and minerals throughout the plant.

Vascular plants, also called tracheophytes, contain two types of vascular tissues, xylem and phloem:

Xylem transports water and minerals from the roots to the plant's tissues. The walls of xylem are strengthened by lignin.
Phloem transports sugar to a plant's tissues. It also transports soluble organic products formed as a result of photosynthesis.
The roots of vascular plants anchor them to the ground and help in absorption. Evolutionary evidence shows that root tissues of today's plants are like the stem tissues of earlier plants. Hence, it's possible that roots have evolved from lower parts of stems or that, as the layers of soil grew, stems were covered by soil and evolved into what we know as root tissues.

With the help of roots to anchor them firmly to the ground and lignified vessels enabling transport of water and nutrients, vascular plants grow to incredible heights and outcompete other plants for sunlight.

Vascular plants evolved from bryophytes, which, as you may recall, are nonvascular plants. Bryophytes were the first land plants that were dependent on water for reproduction. Since these plants do not have vascular tissues, they absorb water by osmosis. Bryophytes include mosses, liverworts, and hornworts.

Bryophytes and vascular plants share certain traits. For instance, both bryophytes and vascular plants show alternation of generations during reproduction. During evolution, there were alterations in common features and development of some new characteristics. These new characteristics were:

reduced gametophyte and dominant sporophyte
evolution of vascular tissues
evolution of leaves and roots
The evolved plants with these features are called tracheophytes. Thus, bryophytes, or nonvascular plants, evolved into tracheophytes, or vascular plants. Let's study the features of tracheophytes using ferns and horsetails as examples.

Early vascular plants included ferns, which have evolved to form more than 10,000 species worldwide. The ferns belong to the phylum Pteridophyta and are known as pteridophytes. The pteridophytes are not fully land–adapted plants as they still need a film of water for fertilization. They can survive only in moist soils.

Ferns have vascular tissues, leaves, stem, and roots. The underground stems are called rhizomes. These stems give rise to large leaves called fronds. Unlike the leaves of bryophytes, which are one cell layer thick, the leaves of ferns are megaphylls: they are large and show multiple venation. They branch into smaller leaflets.

Ferns have a dominant sporophyte generation, during which the plant has two sets of chromosomes. The sporophyte produces spores by meiosis. Almost all fern species are homosporous, which means they produce only one type of spore.

The characteristics of early vascular plants can be summed up as (1) dominant sporophyte,(2) presence of true leaves and roots, and (3) vascular tissues for transport, such as xylem or phloem.

silver fern plant

The horsetail, or Equisetum, is a vascular plant from the arthrophyta group. Like ferns, horsetails can survive only in moist soils as they need water for fertilization. Thus, even horsetails are not fully adapted land plants.

Horsetails have spine–shaped leaves. While these leaves are not like the broad megaphylls of vascular plants, they are also not the primitive, small, microphyll leaves that have a single vein. Leaves of horsetails are recognized to have evolved by reduction of megaphylls.

The stem is the main photosynthetic organ. Since the stem has joints, horsetails are called arthrophytes, or jointed plants. Some species have two types of stems—vegetative stems, which are green and long, and reproductive stems, which are brown and short. The reproductive stems bear cone–like structures. The stems have air canals that carry gases, such as oxygen, toward the roots.

Horsetails have a dominant sporophyte generation. The cones on the stems produce spores that develop into gametophytes.

The fossilized evidence of certain plants suggested that roots evolved from a part of the plant. Which part?
Evolutionary evidence shows that the root tissues of today's plants are like the stem tissues of earlier plants. Hence, its possible that roots have evolved from lower parts of stems.

Early vascular plants, such as ferns and horsetails, reproduced by forming spores. These plants are categorized as seedless plants. Fossil records suggest that these seedless plants developed during the Silurian period. As these ferns spread on land, they went through an adaptive radiation in the Carboniferous and Devonian periods.Fossil records also show the existence of seed plants around the same time, which was about 360 million years ago.

Seed plants may have evolved from ferns as an adaptation to terrestrial life. Fossil records suggest that these adaptations enabled plants to colonize a wider range of habitats. The most crucial adaptation was the development of the seed. The development of a seed that was resistant to drying and could be dispersed led to the rapid spread of these plants on land.


As seed-bearing plants evolved, speciation occurred more rapidly. You may recall making a cladogram. We can use cladograms to study the evolutionary relationships among plants and trace their cladistic relationships.

The tracheophytes, or vascular plants, comprise the largest plant group. Together they include 93 percent of all land plants. As you can see in this cladogram, seedless vascular plants are represented by two clades, namely, lycophytes and pteridophytes. The seeded vascular plants form the third clade. These seed plants form the majority of living plants on earth.

The seed plants are further divided into two groups, gymnosperms and angiosperms. This grouping is based on whether the seeds are enclosed or naked. The seeds of gymnosperms are naked, which means they are not enclosed within an ovary but instead are presented directly on the surface of the cones. Angiospermic seeds, on the other hand, are enclosed within the tissues of the ovary.

Gymnosperms are the vascular plants that have naked, or uncovered, seeds. Most of the gymnosperms are cone–bearing plants. They carry seeds directly on the cones.

Being vascular plants, gymnosperms have a dominant sporophyte generation. They also show true roots and leaves and have a well–developed system of vascular tissues.

The gymnosperms include four groups: gnetophytes, cycads, ginkgoes, and conifers. Conifers are the most abundant group of cone–bearing plants, which includes trees such as pines, firs, yews, and cedars.

Angiosperms are vascular plants that bear flowers. This group of flowering plants appeared on land about 135 million years ago. Thus, they are the most recently originated plant phylum. They have dominated the earth's landscape due to the presence of their unique reproductive organ, the flower.

The presence of flowers has given several advantages to these plants. The main function of flowers is to attract pollinators. The magnolia is an example of an insect–pollinated plant. Magnolia flowers are pollinated by insects that are attracted by its bright colors. Beetles that earlier pollinated cycad species of gymnosperms moved on to pollinate flowering angiosperms such as magnolias.

The flowers contain ovaries that surround the seeds. Ovaries develop into fruits that protect the seeds and aid in their dispersal.

Which type of plant has seeds protected by ovary layers?
The seeds of angiosperms are covered by the layers of the ovary.

Vascular plants comprise 93 percent of all land plants. They inhabit almost all environments on Earth. Thus, they affect humans in several ways.

Plants play a very important role in the production of oxygen and absorption of carbon dioxide. They help maintain the balance of these gases in the atmosphere.

The roots of large trees hold soil firmly to prevent erosion. This prevents the reduction of the groundwater level and maintains the balance of the water cycle. Plants help maintain greenery and prevent desertification.

Plants are an essential part of the ecosystem. They are the producers of the ecosystem, which taps energy from the sun and provides food to other life–forms. Thus, they form the base of the ecological pyramids. They provide food and shelter to animals and other plants.

Plants protect the earth from the perilous effects of climate change.


Seedless Vascular Plants
Seedless plants such as ferns and horsetails were the first plants to have well–developed vascular systems, roots, and leaves. They carried out photosynthesis rapidly. They led to a decrease in atmospheric carbon dioxide levels. The forests of the first seedless vascular plants decomposed incompletely and led to the formation of coal.

Seed Plants
Seed plants have been providing humans with food for thousands of years. Fossil evidence of squash seeds dating back between 8,000 to 10,000 years has been found in Mexico. The growth of agricultural activity led to the transformation of human societies from cave hunters to settlements dependent on agriculture. For example, early civilizations, such as the Mesopotamian civilization, settled near rivers so they could grow plants. Angiosperms contribute to the planet's biodiversity due to increasing speciation in flowers, which leads to increased bird and insect populations.