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, hoAV, 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 



