The seed-bearing plants
One of the most important stages in the evolution of plants was the development of the seed, as explained in Chapter 2. Reproduction by seeds overcame a number of problems inherent in the terrestrial environment and thereby seed plants became so successful that they crowded out plants without seeds in all of the various land habitats.
At first the seeds were without any special covering; these plants are called gymnosperms and they include the conifers, for example. Later, more advanced plants developed a special covering for the seeds; these are the angiosperms or flowering plants. Clearly the latter had a considerable advantage over the gymnosperms and they ousted them from many floras in the world. However, the gymnosperms by no means totally disappeared and in some habitats, such as the northern hemisphere coniferous forests, the gymnosperms are the dominant plants.
Together with the seed habit, as it is called, went a number of features that contributed. Some of these features appeared in plants before seeds did and as a result these plants are classified by some botanical authorities in a sub-group considered to be ancestral to the gymnosperms. These are the Progymnosperms and examples are given after the statement of the features of the Gymnospermae.
a) The sporophyte generation is usually a tree or a shrub. Various types of branching and leaf shapes exist.
b) Sporangia may be in cones but many other arrangements are to be found.
c) Female gametophyte generation retained within the developing seed, the ovule.
d) Fertilization may be by multiflagellate spermatozoids or by male cells without any special mode of locomotion.
These had a number of the vegetative features of gymnosperms, such as the characteristic wood structure, but did not produce seeds. They are all now extinct.
Examples: Archaeopteris, Protopitys, Aneurophyton, Tetraxylopteris, Rellimia, Triloboxylon.
As has been pointed out in Chapter 2, seed plants appeared relatively soon, geologically speaking, after plants first invaded the land. Seeds are found fossilized in rocks 370 million years old and from then on they became increasingly common. In fact in some strata certain species are so abundant as to give the impression that they were produced by plants dominant in their localities.
Such seeds are given generic names as follows: Archaeosperma, Hydrasperma, Genomosperma, Lyrasperma, Deltasperma.
These early seed plants and similar ones that appear in later rock strata may be placed in the sub-group known as the pteridosperms, the seed-ferns, although not all botanical authorities would necessarily agree with this procedure. It will serve our purposes here, however, as the number of species involved is very large and only an outline account can be given.
Many of these plants had fern-like foliage and, as explained in this chapter, when first discovered they were thought to be ferns. However, as research continued more and more examples were found where seeds were obviously attached to the fern-like foliage.
The pteridosperms have some or all of the following characters:
- Large, spirally arranged leaves on the stems.
- Some of the species have the characters of tree ferns but others had a liane-like or vine-like mode of growth.
- The conducting strands to the leaves are somewhat large in diameter if single, otherwise there may be several strands.
- There is often a large amount of primary wood.
- Secondary wood is common whereas this is very rare in ferns.
- The conducting and supporting cells (tracheids) of the secondary wood had bordered pits on the radial walls.
- The ovules and pollen producing organs were borne on unmodified or only slightly modified foliage. This feature distinguishes the pteridosperms from the conifer sub-group of gymnosperms.
The pteridosperms are now all extinct.
Examples: Lyginopteris, Heterangium, Eremopteris, Callistophyton, Medullosa.
The remaining pteridosperms and other gymnosperms will be dealt with in a box in Chapter 5 as they are more appropriately treated there.
In this history of plants we are now about 280 million years back in time from the present day. For about another 150 million years no fundamentally new types of plants would arise. What we shall see will be the types already established adopting new shapes to leaves and stems, types declining in importance while others contribute more to the floras of the world, types developing new arrangements of sporangia and seeds, some of the latter having new structures.