THE INNER TISSUES OF PLANTS. 273 



part of the stem to bear strains, renders possible an increase 

 of height ; and while after an increase of height the lowest 

 part becomes still further strained, and still further thickens, 

 the part above it, exposed to like actions, undergoes a like 

 thickening. This induration, while it spreads upwards, 

 also spreads outwards. As fast as the rude cylinder of dense 

 matter formed in this way, begins to inclose the original 

 vessels, it begins to play the part of a resistant mass, between 

 which and the outer layers the greatest compression occurs 

 at each bend. While, therefore, the original vessels become 

 useless, the peripheral cells of the developing wood become 

 those which have their liquid contents squeezed out longitu- 

 dinally and laterally with the greatest force ; and, consequently, 

 amid them are formed new sap- channels, from which there is 

 the most active local exudation, producing the greatest 

 deposit of dense matter. 



Thus fusing together, as it were, the individualities of 

 successive generations of plants, and letting that facilitation 

 of the process which natural selection has all along given, 

 be represented by the most favourable working together of 

 these mechanical processes, we are enabled to interpret 

 the leading internal differentiations of plants as consequent 

 on a direct equilibration between inner and outer forces. 

 Here, indeed, we see illustrated in a way more than usually 

 easy to follow, the eventual balancing of outer actions by 

 inner reactions. The relation between the demand for liquid 

 and the formation of channels that supply liquid, as well 

 as that between the incidence of strains and the deposit 

 of substance that resists strains, are among the clearest special 

 examples of the general truth that the moving equilibrium 

 of an organism, if not overthrown by an incident force, must 

 eventually be adjusted to it. 



The processes here traced out are, of course, not to be 

 taken as the only differentiating processes to which the 

 inner tissues of plants have been subject. Besides the chief 

 changes we have considered, various less conspicuous changes 



