54: MORPHOLOGICAL DEVELOPMENT. 



next, that the structure so arising is likely to be main- 

 tained. Such a variation implying, as it does, circumstances 

 especially favourable to the growth of the plant, will give 

 to the plant extra chances of leaving descendants; since 

 the area of frond supported by a given area of the soil, 

 being greater than in other individuals, there may be a 

 greater production of spores. And then, among the more 

 numerous descendants thus secured by it, the variation will 

 give advantages to those in which it recurs. Such a mode 

 of growth having, in this manner, become established, let 

 us ask what is next likely to result. If it becomes the 

 habit of the primary frond to bear a secondary frond from its 

 mid-rib, this secondary frond, composed of physiological 

 units of the same kind, will inherit the habit ; and supposing 

 that the supply of mineral matters obtained by the rootlets 

 suffices for the full development of the secondary frond, there 

 is a likelihood that the growth from it of a tertiary frond, 

 will become an habitual characteristic of the variety. Along 

 with the establishment of such a tertiary frond, as shown in 

 Fig. 74, there must arise a further development of mid-rib 

 in the primary frond, as well as in the secondary frond a 

 development which must bring with it a greater integration 

 of the two; while, simultaneously, extra growth will take 

 place in such of the rootlets as are most directly connected 

 with this main channel of circulation. Without further ex- 

 planation it will be seen, on inspecting Figs. 75 and 76, 

 that there may in this manner result an integrated series of 

 fronds, placed alternately on opposite sides of a connecting 

 vascular structure. That this connecting vascular structure 

 will, as shown in the figures, become more distinct from the 

 foliar surfaces as these multiply, is no unwarranted assump- 

 tion ; for we have seen in compound-leaved plants, how, under 

 analogous conditions, mid-ribs become developed into sepa- 

 rate supporting parts, which acquire some of the characters 

 of axes while assuming their functions. And now 



mark how clearly the structure thus built up by integration 



