THE OUTER AND INNER TISSUES OF PLANTS. 231 



Just noting, for the sake of completeness, tliat in the 

 roots of the higher plants there arises a contrast between 

 outer and inner parts, parallel to the one we have traced in 

 their branches, let me draw attention to another- differentia* 

 tion of the same ultimate nature, which the higher plants 

 exhibit to us — a differentiation which, familiar though it is, 

 gains a new meaning by association with those named above, 

 and makes their meaning still more manifest. I refer to the 

 fact that when, by the budding of axes out of axes, there 

 is produced one of those highly-compounded Phaenogama 

 which we call a tree, the central part of the aggregate be- 

 comes functionally and structurally unlike the peripheral 

 part. On looking into a large tree, or even a small one 

 that has thick foliage, like the Laurel, we see that the in- 

 ternal branches are almost or quite bare of leaves, while the 

 leaf-clad branches form an external stratum ; and all our 

 experience unites in proving that this contrast arises by 

 degrees, as fast as the growth of the tree entails a contrast be- 

 tween the conditions to which inner and outer branches are 

 exposed. Now when, in these most- composite aggregates, 

 we see a differentiation between peripheral and central parts 

 demonstrably caused by a difference in the relations of these 

 parts to environing forces, we get support for the conclusion 

 otherwise reached, that there is a parallel cause for the parallel 

 differentiations exhibited by all aggregates of lower orders — 

 branches, leaves, cells. 



§ 271. Before leaving this most general physiological 

 differentiation, it may be well to say something respecting 

 certain secondary unlikenesses that habitually arise be- 

 tween interior and exterior. For the contrast is not, as 

 might be supposed from the foregoing descriptions, a simple 

 contrast: it is a compound contrast. The outer structure 

 itself is usually divisible into concentric structures. This 

 is equally true of a protophyte and of a phaenogamic axis. 

 Between the centre of an independent vegetal cell and its 



