Ancient Plants / Being a Simple Account of the past Vegetation of the Earth and of the Recent Important Discoveries Made in This Realm of Nature

by Marie Carmichael Stopes

Ginkgoaceæ, e.g. Ginkgo

Fossil and living, dating back, apparently with little change, to Palæozoic times.

We must pay the most attention to the two last groups, as they are so important as fossils, and the Cordaiteæ were a very numerous family in Coal Measure times. They had their period of principal development so long ago that it is probable that no direct descendants remain to the present time, though some botanists consider that the Taxeæ are allied to them.

Of the groups still living it is difficult, almost impossible, to say which is the highest, the most evolved type. In the consideration of the Gymnosperm family it is brought home with great emphasis how incomplete and partial our knowledge is as yet. Many hold that the Araucareæ are the most primitive of the higher Gymnosperms. In support of this view the following facts are noted. They have a simple type of fructification, with a single seed on a simple scale, and many scales arranged round an axis to form a cone. In the microscopic structure of their wood they have double rows of bordered pits, a kind of wood cell which comes closer to the old fossil types than does the wood of any of the other living genera. Further than this, wood which is almost indistinguishable from the wood of recent Araucarias is found very far back in the rocks, while their leaves are broad and simple, and attached directly to the stem in a way similar to the leaves of the fossil Cordaiteæ, and very different from the needle leaves on the secondary stems of the Pine family; so that there appears good ground for considering the group an ancient and probably a primitive one.[10]

On the other hand, there are not wanting scientists who consider the Abietineæ the living representatives of the most primitive and ancient stock, though on the whole the evidence seems to indicate more clearly that the Pine-tree group is specialized and highly modified. Their double series of foliage leaves, their complex cones (whose structures are not yet fully understood), and their wood all support the latter view.

Some, again, consider the Taxeæ as a very primitive group, and would place them near the Cordaiteæ, with which they may be related. Their fleshy seeds, growing not in cones but on short special axes, support this view, and it is certainly true that in many ways the large seeds, with their succulent coats and big endosperm, are much like those of the lower Gymnosperms and of several fossil types. Those, however, who hold to the view that the Abietineæ are primitive, see in the Taxeæ the latest and most modified type of Gymnosperm.

It will be seen from this that there is no lack of variety regarding the interpretation of Gymnosperm structures.

The Gymnosperms do not stand in such an isolated position as do the Angiosperms. Whatever the variety of views held about the details of the relative placing of the families within the group, all agree in recognizing the evidence which enables us to trace with confidence the connection between the lower Gymnosperms and the families of ferns. There are many indications of the intimate connection between higher and lower Gymnosperms. Between the series exist what might be described as different degrees of cousinship, and in the lower groups lie unmistakable clues to their connection with more ancient groups in the past which bridge over the gaps between them and the ferns.

For the present, however, let us confine ourselves to the history of the more important Gymnosperms, the discussion of their origin and the groups from which they may have arisen must be postponed until the necessary details about those groups have been mentioned.

To a consideration of the living families of Araucareæ, Abietineæ, Cupresseæ, and Taxeæ we can allow but a short space; their general characters and appearance are likely to be known to the reader, and their details can be studied from living specimens if they are not. For purposes of comparison with the fossils, however, it will be necessary to mention a few of the principal features which are of special importance in discussing phylogeny.