232 The Living Plant 



structure, — precisely, for example, as chlorophyll has been de- 

 veloped in the leaf, a fibro-vascular cylinder in the stem, and 

 hairs on the roots. Unless our whole philosophy of nature is 

 wrong, there was a time when these things were not: now they 

 are : at some time and in some way meantime they have arisen, and 

 by gradual stages in the course of evolution. Our problem of the 

 origin of the set of the machinery is therefore identical in kind 

 with that of the origin of any adaptation, and thereby is trans- 

 ferred into that separate field of inquiry which forms the subject 

 of our later chapter on Evolution and Adaptation. 



The turning window-plant illustrates very clearly the nature 

 of typical sensitive responses in plants; and all of the more com- 

 plicated cases are identical in principle. Thus, not all stems 

 turn towards hght, for those of wall-climbing Ivies (e. g. the 

 Boston or Japanese Ivy) turn away from it, as manifest by the 

 way in which these plants grow into porches and windows. The 

 advantage, however, is evident on reflection; if these stems 

 turned towards light, like the ordinary sort, they would be car- 

 ried away from the wall and the possibility of clinging thereto; 

 but, turning away from the light, they are flattened up against 

 the wall where their holding discs can secure an attachment. This 

 example shows also that no necessary connection exists between 

 stemness, so to speak, and a set of the growth machinery towards 

 light, but that the set is developed in the organs in correlation 

 with their habits quite regardless of their morphological nature. 

 Again, not all leaves set themselves across the light, for a good 

 many kinds belonging in places very brilliantly lighted, like 

 sub-tropical plains, set their edges to the direction of maximum 

 brightness. In some this position is permanent, and may thus 

 bring the leaves to a vertical north-and-south position, as in the 

 Compass Plant of our prairies, which owes its name to this cir- 

 cumstance; or, the leaves may change their positions, rising 

 from horizontal to vertical at the time of maximum brightness, 

 as in sundry plants of the Pea family (figure 78). The advantage 



