258 PHYSIOLOGICAL DEVELOPMENT. 



the materials assimilated at its fixed and its free ends, it lias 

 no need for a circulation nor, indeed, in the absence of 

 evaporation from any part of its surface, could any active 

 circulation take place. Here, accordingly, the ordinary 

 internal structures are undeveloped : though spiral vessels 

 are not entirely absent, yet they are so rare as to do no more 

 than verify the inference of phaBiiogamic relationship drawn 

 from the flowers. 



The method of agreement, the method of difference, and 

 the method of concomitant variations, thus unite in proving 

 a direct relation between the demand for support and cir 

 culation, and the existence of these vascular woody bundles 

 which the higher plants habitually possess. The question 

 which we have to consider is Under what influences are 

 these structures, answering to these requirements, developed ? 

 How are these internal differentiations caused ? The inquiry 

 may be conveniently divided. Though the supporting tissues 

 and the tissues concerned in the circulation of liquids are 

 closely connected, and indeed entangled, with one another, 

 we may titly deal with them apart. Let us take first the 

 supporting tissue. 



279. Many common-place facts indicate that the me 

 chanical strains to which upright-growing plants are exposed, 

 themselves cause increase of the dense deposits by which such 

 plants are enabled to resist such strains. There is the fact 

 that the massiveness of a tree-trunk varies according to the 

 stress habitually put upon it. If the contrast between the 

 slender stem of a tree growing in a wood and the bulky stem 

 of a kindred tree growing in the fields, be ascribed to differ 

 ence of nutrition rather than difference of exposure to winds ; 

 there is still the fact that a tree trained against a wall has a 

 less bulky stem than a tree of the same kind growing un 

 supported ; and that between the long weak branches of the 

 one and the stiff ones of the other there are decided contrasts. 

 If it be objected that a tree so trained and branches so borne 



