THE INNER TISSUES OF PLANTS. 275 



any active circulation take place. Here, accordingly, the 

 tracheal and mechanical elements are undeveloped. Though 

 spiral vessels are not entirely absent, yet they are so rare as 

 to do no more than verify the inference of phaenogamic rela 

 tionship 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 circu 

 lation, 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 fitly 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 

 have relatively less foliage, and that therefore the^e unlike- 

 nesses also are due to unlikenesses of general nutrition, which 

 may in part be true; there are still such cases as those of 



