Not all plants adapted to life on land in the same ways. One of the major distinctions between groups of plants is the way they transport water and nutrients throughout the plant body. The majority of land plant species have an internal system of interconnected tubes and vessels called vascular tissues. These plants, grouped as vascular plants, will be discussed in the next chapter. Most of the plants you are familiar with - oak trees, roses, grasses and house plants - are vascular plants and have vascular tissue. The other main group of plants, the bryophytes lacked vascular tissues. These nonvascular plants transport water and nutrients by osmosis and diffusion, much as algae do.
The earliest fossils of vascular plants are about 400 million years old, but the earliest fossils of bryophytes are only about 350 million years old. For this reason, some scientists claim that bryophytes developed from vascular plants that gradually lost their vascular tissues. However, other scientists claim that bryophytes evolved independently. If many early bryophytes decomposed before they were fossilized, perhaps bryophytes developed earlier than the fossil record indicates.
Modern land plants probably evolved green algae. About 400 million years ago the sea was teeming with life, but the land was mostly barren rock. Algae and other marine organisms began to grow near shore because of the availability of direct sunlight and of minerals washed off the shore. As competition for resources in the sea increased, some of the algae started growing on land.
Plants evolved structures that produce sperm that can travel through the air, in the wind or on the bodies of insects or animals. Many plants developed multicellular reproductive structures to protect the developing zygote and keep it from drying out.
About 400 million years ago green algae-like organisms evolved these structures necessary to survive on land-cuticles, stomata, lignified cells and multicellular reproductive organs. Plants then colonized the land and exploited a habitat where they had no competitors.
More than 250000 species of vascular plants have been identified. Vascular plants have adapted to almost every kind of climate. Their complex adaptations also give them advantages over other kinds of plants. These advantages allow vascular plants to grow and flourish where vascular plants cannot.
The plants in turn influence their environment. Wherever vascular plants flourish, they create habitats and provide food for various species of insects as well as for many other kinds of animals.
Like nonvascular plants vascular plants are through to have developed from water-dwelling green algae approximately 400 million years ago. The tracheophytes, or vascular plants, share some basic adaptations on land with the bryophytes. Both groups have a waxy outer cuticle that allows an exchange of gases. Multicellular reproductive structures that protect delicate zygotes evolved in both vascular and nonvascular plants.
Unlike bryophytes tracheophytes have an internal network of tubes known as the vascular system. The tubes carry water, nutrients and the products of photosynthesis throughout the plant. The vascular system can transport fluids over long distances, from roots buried deep in the soil to treetops perhaps hundreds of meters above the ground. The cell walls of tracheophytes contain lignin, a substance that helps to support the plant body.
Vascular system.The body of a vascular plant is made up of three types of structures. The roots absorb moisture and nutrients from the soil and anchor the plant. Leaves have chloroplasts and produce food by photosynthesis. The stem contains vascular tissues that transport substances between the roots and leaves and support the leaves. Because they contain vascular tissue, tracheophytes are said to have true roots, stems and leaves.
The vascular system includes two distinct kinds of vascular tissues. The xylem transports water and minerals absorbed by the roots up to those parts of the plant that are above the ground. The phloem carries sugar and other soluble organic materials produced by photosynthesis from the leaves to the rest of the plant.
Seedless vascular plants.Seedless vascular plants include living representatives from four phyla: Psylophyta, the whisk ferns; Sphenophyta, the horsetails; Lycophyta, the club mosses and their relatives; and Pterophyta, the ferns. Seedless vascular plants reproduce sexually by means of flagellated sperm and need water to swim to the egg cells. The sporophyte generation produces haploid spores that develop into small, independent gametophytes.
Reproduction in vascular plants.The life cycles of vascular plants are differed from those of nonvascular plants. In vascular plants the sporophyte is the dominant generation. The sporophyte is physically larger, shows more complex development and produces more varied types of cells then the gametophyte.
Tracheophytes are traditionally divided into two groups: seedless plants and seed plants. Seedless plants developed first and still have traits that show their watery origin. Most seedless plants require water for sexual reproduction. Seed plants developed and important adaptation; they reproduce sexually by forming seeds.
Adaptations in vascular plants.Certain adaptations in tracheophytes gave them important evolutionary advantages over nonvascular plants. Consider for example the different ways in which bryophytes and vascular plants cope with an environment where the topsoil permanently frozen. To survive in this environment a plant must have mechanisms to overcome the lack of available water. Nonvascular plants cannot easily tolerate these conditions. Bryophytes need an abundant supply of unfrozen water to transport materials by osmosis and diffusion and to reproduce sexually. However, tracheophytes are able to obtain the water they need from roots growing beneath the frozen surface. The sperm of vascular plants, protected in pollen, can reach egg cells through the air. Seeds from seed plants can remain dormant until conditions are favorable for their germination. These adaptations enabled tracheophytes to survive in unfavorable conditions and contributed to the worldwide proliferation of vascular plants.
The seed plants reproduce chiefly by forming seeds. Every seed contains a plant embryo, or partially developed plant that is capable of growing into a mature plant. A seed also contains one or two embryonic leaves called cotyledons. Cotyledons may became the young plant’s first leaves or may be used as food supply as the plant grows. A hard covering called the seed coat encases the embryo and cotyledons and protects them from physical injury and drought.
The development of seeds greatly increased the ability of tracheophytes to survive in unfavorable conditions. Seeds protect plant embryos from harsh conditions. As a result, embryos can lie dormant for years and still produce healthy plants when conditions allow. Some seed-hearing structures including stickers, burrs, and thistledown, travel long distances on animals or in the wind. In this way seeds substitute for mobility, allowing vascular plant species to spread to new areas.
There are two groups of seed plants developed from early vascular plants. Gymnosperms produce their seeds in cones and generally keep their leaves throughout the year. Angiosperms produce flowers, bear their seeds in fruit and in general lose their leaves annually. Like all tracheophytes the dominant generation in gymnosperms and angiosperms is sporophyte. The roots, stems and leaves of the sporophyte make up the plant’s vegetative body. The leaves and stems carry out photosynthesis and normal growth.