Division of Pteridophytes Plants
- Psilophyta (Whisk ferns): Homosporous, motile sperms, no differentiation between root and shoot, two genera and several species
- Lycophyta Lycopods (including clubmosses): Homosporous or Heterosporous motile sperm, four genera and about 1000 species
- Sphenophyta (Horsetails): Homosporous, motile sperm, one genus and 15 species
- Pterophyta (Ferns): Homosporous, a very few heterosporous, motile sperms about 12,000 species
- Coniferophyta (Conifers): Heterosporous, seed-forming, sperm-lack flagella, about 50 genera, with about 550 species
- Cycadophyta (Cycads): Heterosporous, seed forming, sperm is motile but carried to the vicinity of the egg by a pollen tube, ten genera and about 100 species
- Ginkgophyta (Ginkgo): Heterosporous, seed-forming, sperm motile but carried to the vicinity of the egg by a pollen tube. Deciduous tree with fern-like leaves and seeds resembling that of a small plum, with a fleshy, ill-scented outer covering, one species
- Gnetophyta (Ephedra, Gnetum, Welwitschia): Heterosporous, seed forming sperm not motile, shrubs, vines, three diverse genera and about 70 species
- Anthophyta (Flowering plants, or angiosperms): Heterosporous, seed-forming, sperms not motile, range from tiny plants to the largest trees known, about 235,000 species
A seed is a developing sporophyte individual surrounded by a tough protective coat whose embryonic development has been temporarily arrested. Seeds are produced by plants to ensure the perpetuation of their types and for the spread of the species to newer areas. From the morphological point of view, the outer layers of a seed consist of tissues derived from the parent sporophyte on which it occurs; inside are the derivatives of the megasporangium of that sporophyte, enclosing the derivatives of the gametophyte, and, within them, the young sporophyte of the next generation, the embryo.
Let us see how seeds are produced. Microspores, which are produced within the microsporangia as a result of meiosis, ultimately differentiate into microgametophytes known as pollen grains. These are shed by the parent plant and transported to the vicinity of the megagametophyte by the pollen tube. When it reaches the egg, fertilisation takes place and the resulting zygote begins to divide eventually growing into an embryo. Around the embryo and the megagametophyte the seed coat becomes mature and more or less hard or tough, depending on the particular kind of the plant. Germination occurs when there is a sufficient supply of water and other conditions are appropriate. The seed coat splits and the embryo resumes the process of cell-division and eventually grows into an adult sporophytic plant.
The seed is a crucial adaptation to life on land because it protects the embryonic plant from drying out or being eaten up, when it is at its most vulnerable stage. Most kinds of seeds have abundant food stored in them either inside the embryo or in a specialised storage tissue. This food is used as a ready source of energy by the rapidly growing young plant, playing the same role as the yolk of an egg. The evolution of the seed clearly was a critical step in the domination of the land by plants.
Of the nine divisions of vascular plants listed above, the living representatives of the Psilophyta, Lycophyta, Sphenophyta, and Pterophyta do not have seeds, but the rest of the vascular plants have. The four divisions of living seed plants (Conifers, Cycads, Ginkgo and Gnetophytes) in which the ovules are not completely enclosed by the tissues of sporophytic individual on which they are borne at the time of pollination are called gymnosperms (from Greek sperma meaning seed and gymnos meaning naked, in other words 'naked-seed' plants).
Flowering plants (division Anthophyta) differ from other seed plants in that their ovules are enclosed within the tissue of the parent sporophyte in structures called the carpel. Because of their enclosed ovules and seeds, flowering plants are called angiosperms (Gr., angion meaning vessel, i.e. carpel, and sperm meaning seed). The carpel ultimately develops into a fruit which encloses the mature seeds.