THE OUTER ASD IXXER TISSUES OF PLANTS. 231 



Just noting, for the sake of completeness, that 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 Phocnogams 

 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 

 clemonstrably 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 phaonogamic axis. 

 Between the centre of an independent vegetal cell and ita 



