50 



MORPHOLOGICAL iJEVELOPMENT. 



on its upper surface, and having a sligh.tly> 

 marked mid-rib and rootlets. And sup- 

 pose that, as shown, a secondary' frond ia 

 njj. - - proliferously produced from the mid-rib, 

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

 the ordinary discontinuous development, 

 can thus become a continuous development, 

 only on condition that there is an adequate 

 supply, to the secondary frond, of such 

 ' materials as are furnished by the rootlets : 

 the remaining materials being obtainable 

 by itself from the air. Hence, that portion 

 of the mid-rib lying 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, 

 7^ 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, among 

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

 tion 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 tho 

 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 



