Origin of Seed Habit of Plants | Pteridophyta | Botany

Origin of Seed Habit:

The dominant plants of the present day are the Gymnosperms and the Angiosperms. It seems certain that the dominance of these groups of plants over the vascular cryptogams, which reproduce by means of spores, is mainly due to their acquisition of the method of reproduction by seeds. In the cryptogamic type of reproduction, the spores, after liberation from the sporangium of the parent plants, produce very delicate sexual plants,—the gametophytes.

The survival of these gametophytes, as well as their sexual repro­duction, are mainly dependent on the presence of moisture, and as such is a matter of chance. In the seed plants, however, the spore, which produces the mega-gametophyte is retained within the sporangium, and once the male spore, which produces the micro-gametophyte, comes in contact with the micropyle or the stigma.

Further developments and fertilization are carried out within the integuments and are not at all dependent on environmental factors.

The process of the evolution of seed is thought to have com­menced with an ancient homosporous form, which has given rise to the heterosporous condition. Most botanists regard the heterosporous condition as characteristic of all seed plants. The pollen is presumed to develop from a microspore, while the embryosac from the megaspore.

The occurrence of many comparatively small spores within the mega-sporangia in Selaginella suggests a condition possibly not far advanced beyond the homosporous state. On the other hand, the sporangia with a fewer but comparatively larger spores, indicate an advance in the direction of seed habit.

Further, Selaginella rupestris and S. apus have progressed a long way in the direction of seed habit, as the megaspore in these cases is retained within the mega-sporangium, and the megaspore, while still within the sporangium, develops archegonia and forms embryo with roots and cotyledons.

Thus, Selaginella is considered by many morphologists to illustrate the transformation from the strictly cryptogamic condition to the seed habit, that is, it shows the replica of stage through which the ancestor of the seed passed during its process of becoming a seed.

Thomson (1927, 1931, 1934) from his comparative studies of the sizes and other characteristics of pollen spores and seed spores concluded that seed plants are heterosporous and have evolved from homosporous ancestors, and it is the heterothally and not heterospory that constitutes the important feature of the seed plants.

Halle (1935) considered the terminal position of the primitive cupulate seed of the early lower carboniferous age, as significant and derived them from the terminal spore cases of the Psilophytales.

As long ago as 1904 Miss Bensen suggested that the cycadofilicinean seed may be interpreted as a synangium in which except­ing a single sporangium, all are sterile. This means that the ‘seed state’ was preceded by one in which a sporangium was closely encircled by a ring of sporangia, and these sporangia became sterile and formed the integument by fusion.

Walton has very recently simplified this concept and suggests that the encircling elements were not sterile sporangia, but sterile branch tips. There are many fossilized plant remains known to support Walton’s hypothesis.