70 MORPHOLOGICAL DEVELOPMENT. 



portion of the axis below it. And now observe how, when we 

 take this for the unit of composition, the metamorphoses 

 which the phtenogamic axis displays, are inferable from known 

 laws of development. Embryology teaches us that arrest 



of development shows itself first in the absence of those parts 

 that have arisen latest in the course of evolution ; that if 

 defect of nutrition causes an earlier arrest, parts that are of 

 more ancient origin abort ; and that the part alone produced 

 when the supply of materials fails near the outset, is the prim- 

 ordial part. "We must infer, therefore, that in each seg- 

 ment of a PhaBnogam, the foliar organ, which answers to the 

 primordial frond, will be the most constant element ; and 

 that the internode and the axillary bud, will be successively 

 less constant. This we find. Along with a smaller size of 

 foliar surface implying lower nutrition, it is usual to see a 

 much- diminished internode and a less-pronounced axillary 

 bud, as in Fig. 124. On approaching the flower, the 



\ 



axillary bud disappears ; and the segment is reduced to 

 a small foliar surface, with an internode which is in most 

 cases very short if not absent, as in 125 and 126. In the 

 flower itself, axillary buds and internodes are both want- 

 ing: there remains only a foliar surface (127), which, 

 though often larger than the immediately preceding foliar 

 surface, shows failing nutrition by absence of chlorophyll. 

 And then, in the quite terminal organs of fructification (129), 

 we have the foliar part itself reduced to a mere rudiment. 

 Though these progressive degenerations are by no means 

 regular, being in many cases varied by adaptation to par- 

 ticular requirements, yet it cannot, I think, be questioned, 



