244 



THE SPORE-PRODUCING ORGANS 



[CH. 



a form according naturally, as in Angiopteris, with the gentle convexity of 

 its massive form. But in smaller sporangia, where the initial convexity is 

 greater, the anticlinal walls converge, and the archesporium has the form of 

 a three-sided pyramid {e). Here the wall {x, x) is inserted on an inner 

 periclinal ; but in {d), which represents the segmentation in ScJiizaea, Tri- 

 cJioinanes, or Thyrsopteris, the wall {x, x) cuts another anticlinal. This marks 

 another step in attenuation of the sporangium, though only a slight one, and 

 in other respects the segmentation of (d) resembles that of the simplest 

 sporangia oiOsnuinda or Todea. The segmentation of the young sporangium 

 common for the Gradatae is shown in Fig. 238, b^ c. The latter is seen in the 

 Cyatheaceae; the former is a slight variant also seen in them, and in Cerato- 

 pteris. In the more advanced Polypodiaceae, however, where the sporangium 

 is often long-stalked, the wall cut by the wall {x,x) may be no longer inclined 



Fig. ■238. Diagrams illustrating the segmentation of sporangia of various 

 Ferns. a = Polypodiaceae (compare Kny, Wandtafeln xciv); 

 b = Ceratopteris (compare Kny, Parkeriaceen, Taf. XXV, Fig. 3); 

 c — Alsophila; d=Schizaea, or Thyrsopteris, or Trichomanes; 

 e, f= Todea ; g= Angiopteris. 



to the axis of the sporangium, but transverse (Fig. 238, <?). From this series 

 of diagrams it is seen how gradual are the steps from the segmentation typical 

 of Eusporangiate Ferns to that extreme simplicity to be found among the 

 Leptosporangiates. There is in fact no strict phyletic barrier between the 

 two types ; the difference is so gently graded over that the unity of the scheme 

 seems clear. The initial segmentation of the sporangium may itself be held 

 as an index of the progressive attenuation of the sporangium in terms of 

 Descent. It will be shown that it goes along with attenuation of the sporangial 

 stalk, simplification of the sporangial head, and finally with the progressive 

 reduction of the individual productiveness of the sporangium, as measured 

 by the spore-output of each of them. 



In such cases as those of Ceratopteris, Alsophila, and Schizaea (Fig. 238, 

 b, c, d), a many-rowed stalk is initiated by the first segmentations. But in 



