Seed 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.

4.1. Bryophytes and Mosses

4.1.1. Bryophytes

Almost all bryophytes are small plants, ranging height from 1 to 20 cm (0.3 to 8 in.). Because gravity restricts the processes of osmosis and diffusion, nonvascular plants grow close to the ground. Bryophytes also lack the rigid tissues that vascular plants have to support vertical growth. Some bryophytes, however, grow to a large size. Most are aquatic species that live in rivers and streams. Supported by the buoyancy of water, these aquatic bryophytes can grow larger than terrestrial species,

Because they do not have vascular tissues, bryophytes do not possess true roots, leaves, or stems. What appear to be "roots" and "leaves" in bryophytes are not specialized structures like those of vascular plants, but mere elongations of the "stem". The roots of vascular plants anchor the plant body and absorb water from the soil. Rootlike rhizoids in bryophytes perform the same functions but do not channel water to other parts of the plant. The upper parts of bryophytes obtain moisture through their leaves, which absorb water through pores. Bryophytes leaves are usually only two cells thick.

The life cycles of bryophytes exhibit alternation of generations. The gametophyte (n) produces gametes (n) by mitosis. During fertilization, the gametes fuse. The resulting zygote grows into the sporophyte (2n), which produces spores (n) by meiosis. When these spores germinate, they develop into the new gametophyte (n) generation. In all bryophytes, the gametophyte is the dominant form. In other words, the gametophyte is the green leafy plant that makes up the major portion of the organism’s life cycle. The phylum Bryophyta is grouped into three classes. Over 9,500 species are included in the class Muscopsida, the mosses. About 6,000 species of liverworts belong to the class Hepaticopsida. The smallest class, Antherocerotopsida, has about 100 species. Members of this class are commonly called hornworts.