THE USES AND 



LEAVES IN PLANTS. 407 



■-• 





the basis of the development of more special forms? Before considering this 

 question, however, I will consider what other resources of explanation hypothe- 

 sis can command. The spiral arrangement might be supposed to be the result 

 of a physiological necessity among the laws of growth, through which single 

 leaves would be produced at regular intervals or steps of development, and 

 placed so as to compass the utilities we have already considered, namely, those 



of horizontal and longitudinal distribution 



and vertical allig 



ment in remoter ones. This would account for the spiral arrangements, and it 

 may be a superior mode of growth, or involve some physiological utility; but 

 that it is not a necessity, is proved by the arrangements of the whorl, in 



which all the members of a group of 



dy produced. The 



existence of the whorl, then, sets this hypothesis aside. Again, we might sup- 



■ 



pose on the theory of types that these two great types of arrangement are 

 two fundamental facts in the higher vegetable life, parts of a supernat- 

 ural plan ; two aboriginal and absolute features in this plan. But this, as we 

 have seen, is not to solve the problem, but to surrender it; or rather to 

 demand its surrender, and forbid its solution. Again, the production of adven- 

 titious buds in plants, or in separated parts of plants, as in cuttings, dependent 

 only, apparently, on a favorable situation for nutrition, is of common occurrence 

 even in the higher plants. If we could suppose that the definite horizontal 

 distributions of successive leaves were wholly superseded in their utility by the 

 distributions along the stem, or that the leaves could thus be sufficiently 

 exposed to light and air; the. power of the adventitious production of buds 

 or leaves in favorable situations might have caused an arrangement without 

 this feature of spiral regularity. But they would still be brought into vertical 



alignments, if the physiological advantage of the simpler cycles, which has 

 been pointed out, be a real and effective one; for even the so-called adven- 

 titious production of buds may reasonably be supposed to be governed by 

 supplies of nutriment. Moreover, these vertical lines would be placed at equal 

 intervals around the stem, on account of the advantage there would be in such 

 a distribution, both for • internal and external nutrition. But though leaves would 

 thus be placed at convenient distances along equidistant vertical lines, there 

 would be no consideration of utility to govern their relations to each other 

 on different lines, so as to throw them into whorls, or into definite spiral 

 arrangements. It mi<rht. however, be advantageous for leaves on a line between 



fe^"^" 10 ' •*" ""g 



two others to be placed in intermediate positions with respect to the leav 



