2O6 



SYMMETRY OF THE SPOROPHYTE 



bving it about has led Goebel to the conclusion that unequal illumination 

 is a determining cause ; for he found that in Diphystium 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. Haberlandt 

 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 symmetry of the shoot in 

 Vascular Plants their embryology will be put temporarily aside, and it 

 will be considered towards the close of this chapter. Questions of 

 symmetry in Vascular Plants are also complicated by the presence of a 

 foliar development. This difficulty will weigh most with those who 

 entertain some phytonic theory of the shoot ; but into their difficulties 

 we need not enter, since reasons have been given for not sharing their 

 view (Chapter XL). Assuming, in accordance with our earlier discussions, 

 a strobiloid theory, the shoot will be habitually regarded as an entity, 

 and its symmetry as a whole will be held to be determined by the 

 equal or unequal development of the appendages, with or without a 

 corresponding development of the axis which bears them. 



FIG. 104. 



Diphysciutn foliosum. Longi- 

 tudinal section of a stem bearing 

 a sporogonium. The arrow indi- 

 cates the prevalent incidence of 

 light. (After Goebel.) . 



