VI] PROGRESSIVE SIMPLIFICATION OF STRUCTURE 117 



genus Selaginella the upright types, which on grounds of comparison are 

 held to be relatively primitive, and live in exposed situations, have a rela- 

 tively robust construction. 5. spinidosa has from the first a stem-apex with 

 a plurality of initials {Land Flora^ Fig. 190). The dorsiventral forms are 

 specialised for growth under forest shade. Of these 5. Wallichii holds an 

 intermediate position with two initials, but 5. Martensii has only a single 

 initial at the apex of the stem. A similar progressive simplification in rela- 

 tion to shade probably holds for the main series of the Filicales. The relatively 

 primitive Eusporangiatae with their robust habit are characteristic of those 

 early times when life for them was necessarily exposed. They shared their 

 massive sporangia with the Eusporangiate Lycopods, Sphenophylls, Cala- 

 marians, Psilotales, and Psilophytales, all of which lived under conditions 

 substantially alike. As vegetation progressed forest shade became more 

 efficient. Shade-loving plants, such as the Selaginellas, the Ferns, and Mosses, 

 became fined down in texture in relation to it. They developed a thinner, 

 sometimes even a filmy structure: their leaf-area was enlarged, and in Ferns 

 it became reticulate: the sporangia grew less robust: and with these changes 

 there advanced that progressive simplification of structure which finds its 

 reflection in the more delicate and exact segmentation of their apical regions, 

 and of their sporangia. If this be the true story, then exactitude of segmen- 

 tation and the presence of a single initial cell are not necessarily signs of a 

 primitive state, as has been commonly assumed; but a derivative condition, 

 secondarily acquired as a consequence of life under new circumstances where 

 a delicate construction proves practically efficient. 



There is reason to believe that when the sporophyte took possession of 

 the land, it soon assumed a relatively robust structure fit to meet ordinary 

 subaerial conditions. This was provided by a complex primary segmenta- 

 tion, perhaps ultimately derived from a transversely segmented filament, by 

 crosswise division, thus giving four initials. In many cases this has persisted, 

 and it is seen to-day in the sporogonia of many Liverworts, such as Jmtger- 

 inannia, and Anthoceros: in the embryos of Lycopods, of some Eusporangiate 

 Ferns, and of Angiosperms. In certain of these the same construction is 

 perpetuated in the growing tips, as in Eusporangiate Ferns and some Lyco- 

 pods. But in others a simplification led to more exact segmentation at the 

 apex with a single initial. That is seen to-day in the Leptosporangiate Ferns, 

 the advanced Selaginellas, the stems and roots of the Equiseta, and the 

 sporogonia of the Bryineae. Signs of it are also seen in the apices of some 

 aquatic Angiosperms, for instance in the roots of Eleocharis pahistris, as 

 demonstrated by Schwendener. 



This is not the place to discuss fully the problem of what it is that deter- 

 mines the singularly regular methods of segmentation so beautifully illustrated 

 in the apices of the Filicales. For us the point is that it presents features 



