130 PLANT LIFE 



with began to differentiate mechanical tissue, 

 appropriate both in form and position to the 

 particular forces it became necessary to 

 counteract. 



In another connection, it may be observed 

 that new vascular tissue can be differentiated 

 in the leaves of some plants if their vascular 

 bundles are injured, and this new tissue is 

 formed at the expense of cells which hitherto 

 have discharged other and very different 

 functions. The union of appropriate tissues 

 between stock and scion in grafting furnishes 

 yet another example. These instances have 

 been mentioned here to avoid giving too one- 

 sided an impression of the evidence available 

 in connection with the problem. 



Such experiments as those above mentioned 

 serve to throw a little light on the matter, 

 by enabling us to realise that the final result 

 is due not so much to a process of direct 

 adaptation as to the interaction of a number 

 oi different functions. These have somehow 

 or other to be correlated within the plant, in 

 order to produce the observed effect. Nutri- 

 tion obviously plays a part, though how large 

 or important it is we do not know; but at 

 least it is essential, if only as providing the 

 means for thickening the cell walls. It is, 

 however, very clear that the causes under- 

 lying the adaptive character of the distribution 

 of the tissues are still far to seek, and much 

 more detailed analysis of the life processes 

 are required before we shall be able to trace, 



