THE PLANT SLOWLY EMERGES AS A COMPLEX STRUCTURE

IN the simplest Devonian plants the fertile part was merely a terminal sac containing spores—the differentiation of the axis into sterile and fertile parts seems to have preceded the evolution of more or less specialised organs like roots, stems or leaves. The terminal spore-sacs were sometimes borne singly and sometimes in clusters, and from the latter it is possible to derive the cone, which is simply an aggregation of spore-sacs on a specialised branch, and may be with or without interspersed bracts or leafy structures.

In the fern type the spore-sacs are often borne on ordinary fronds, but here again we must remember that these fronds are themselves probably modified branch systems. The spores were at first all exactly alike, but in various Palaeozoic groups of plants one soon finds that two kinds of spores are produced, differing in size. One of these doubtless produced the male cells, and the other the female. Here it is important to bear in mind the ‘alternation of generations ‘1 so characteristic of ferns and their allies as well as of mosses and liverworts. In the latter the ‘plant ‘is the sexual generation, and the spore-producing generation is relatively unimportant; in the ferns, and in all the other fossil land-plants from Rhynia onwards, we have to do with the spore-producing generation, nothing being known of the generation which bore the sexual organs.

In the higher plants the sexual generation is much reduced, and is not an independent body. When the number of ‘female ‘spores in each sac is reduced to one, and this after fertilisation remains attached to the parent plant and is provided with a covering, we have an approach to the seed. The origin of the seed-habit is, however, not yet solved; it arose independently in different groups of Carboniferous plants, and some Carboniferous seeds were considerably more complex than any seeds of to-day.

We cannot deal in any detail with the innumerable variety of spore-sacs (sometimes borne on special branches, sometimes on ordinary fronds), cones and seeds found in the coal measures and the other deposits of this era, but we must emphasise that all these organs are usually found detached. Moreover, some very seed-like bodies have turned out on

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microscopical examination to have been spore-bearing organs, and vice versa. Every scrap of evidence which will serve to relate one set of organs to another, or to throw light on structural details, is therefore of value, and the amateur collector should keep a sharp look-out for seeds or spore-sacs attached to stems or fronds.

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