. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 206 SYMMETRY OF THE SPOROPHYTE bring it about has led Goebel to the conclusion that unequal illumination is a determining cause; for he found that in Difihyscium the flattening of the unilateral sporogonia always takes place on the illuminated side (Fig. 104). The primary advantage which is gained by the dorsiventral development is the enlargement of the assimilating system. Haberlandl has shown how considerable the assimilatory activity is in the capsule of Mosses, and has specially pointed out in th
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 206 SYMMETRY OF THE SPOROPHYTE bring it about has led Goebel to the conclusion that unequal illumination is a determining cause; for he found that in Difihyscium the flattening of the unilateral sporogonia always takes place on the illuminated side (Fig. 104). The primary advantage which is gained by the dorsiventral development is the enlargement of the assimilating system. Haberlandl has shown how considerable the assimilatory activity is in the capsule of Mosses, and has specially pointed out in the case of Buxbaumia how much more extensive, as well as better stocked with chloroplasts, the enlarged face of the capsule is, than is the side directed downwards. A secondary advantage is that the oblique position is effective in connection with the scattering of the spores. Such facts relating to the Bryophyta clearly indicate that the radial type of construction is the fundamental one for their sporogonia. Not only are the departures from that type relatively few, and far from being extreme examples as compared with dorsiventrality elsewhere, but also they may in some cases at least be put in definite relation with external causes, and the altered form be shown to have a favourable biological effect. When to this it is added that the dorsiventrality appears comparatively late in the individual development, the case seems fully made out for the priority of the radial construc- tion of the sporogonium of Bryophytes. The infinitely greater variety of form among the Vascular Plants in some measure confuses the ^question of a fundamental type of symmetry for them. Moreover, the issue is further obscured by the diversity of their embryogeny: so long as the initial characters of their embryos are held accurately to reflect their evolutionary story, this difficulty will remain, but in a previous chapter this doctrine has been held open to doubt. In the present discussion of the symm
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