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 
of 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, 
