50 



MORPHOLOGICAL DEVELOPMENT. 



on its uj)i3er surface, and having a sliglitlv* 

 7j marked mid-rib and rootlets. And sup- 

 pose that, as shown, a secondary frond is 

 ^Mh ^ proliferousl}^ produced from the mid-rib, 

 i ^'^^"^ and continues attached to it. Evidently, 

 the ordinary discontinuous development, 

 can thus become a continuous development, 

 cnly on condition that there is an adequate 

 supjjly, to the secondary frond, of such 

 materials as are fiu-nished by the rootlets : 

 the remaining- materials beino' obtainable 

 by itself from the air. Hence, that portion 

 of the mid-rib Ij^ing between the secondary 

 frond and the chief rootlets, having its 

 function increased, will increase in bulk. 

 .An additional consequence will be, a 

 greater concentration of the rootlets — 

 there will be extra growth of those which 

 are most serviceably placed. Observe, next, 

 that the structure so arising, is likely to be 

 maintained. Such a variation implying, 

 as it does, circumstances especially favour- 

 able to the growth of the plant, will give 

 to the plant extra chances of leaving de- 

 scendants ; since the area of frond sup- 

 ported by a given area of the soil, being 

 greater than in other individuals, there 

 may be a greater production of spores. And then, auiong 

 the more numerous descendants thus secured by it, the varia- 

 tion WT.11 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 suppoB^'^ig 



