ii8 CELLULAR CONSTRUCTION [CH. 



that may be used in phyletic comparison. But it is worthy of note in this 

 connection that there is no obligatory correspondence between segmentation 

 and the genesis of parts, or of tissues. It is true that there may be such 

 correspondence in special cases. For instance Kny has shown how in Cerato- 

 pteris one leaf is produced from each segment of the apical cell (90). Klein 

 states that one leaf springs from each of the two dorsal segments in the rhizome 

 of Polypodunn vulgare, though none arises from the ventral segments (94). 

 The subdivision of the segments of the root in many Ferns are found to 

 correspond exactly to the limits of the stele. These facts appear to suggest 

 habitual correspondence. But the suggestion is negatived by the fact that 

 the lobes of the leaf of Ceratopteris do not correspond to the segments of the 

 apical cell of the leaf. Further, in Leptosporangiate Ferns the wings of the 

 leaf are formed from the middle region of each of the two rows of segments : 

 in the Osmundaceae they each arise from the margins of only two of the 

 three rows of segments. Lastly, in the sporangia of ordinary Leptosporangiate 

 Ferns three lateral segments are cut off from the sporangial mother-cell: but 

 in Dipteris and Metaxja, and in Cheiroplenria, as will be shown later, there 

 are only two segments instead of the usual three. Yet the structure and 

 orientation of the annulus remains the same as it is in the allied Platycerium, 

 and indeed in most other Leptosporangiate Ferns, where there are three. 

 These examples show that correspondence between segmentation and mature 

 structure may exist, but that the two are not necessarily related. Segmenta- 

 tion would thus appear to be a phenomenon independent causally of the 

 genesis of parts and tissues, though frequently the two may be related. 



Returning in conclusion to the main subject of this Chapter, the final 

 decision on the point of the priority of the Eusporangiate or of the Lepto- 

 sporangiate Ferns in Descent will have to be based on the whole sum of 

 knowledge respecting them. All their parts will have to be treated com- 

 paratively in testing its truth, and to this a large part of the present volume 

 will be devoted. Pending the completion of that study the view is here 

 adopted as a provisional hypothesis, based both on comparison and upon 

 the palaeontological record, and harmonising with the facts of cellular con- 

 struction of the plant-body as described in this Chapter, viz. that the Eu- 

 sporangiate Ferns are relatively primitive, and appeared first: and that the 

 Leptosporangiate Ferns are derivative and specialised forms, and essentially 

 shade-loving: and that they appeared later in the course of Descent. While 

 many of the former have survived to the present day, the Leptosporangi- 

 atae are the typical Ferns of modern times. The provisional hypothesis in 

 fact implies that there has probably been a progression from a more robust 

 type of cellular construction as seen in the Eusporangiate Ferns to a less 

 robust type as seen in the Leptosporangiatae. 



