THE OUTER AND INNER TISSUES OF PLANTS. 249 



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. Each great 

 plant shows it. When, by the budding of axes out of axes, 

 there is produced one of those highly-compounded Phasnogams 

 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 

 which 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 paral- 

 lel 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 which usually arise between 

 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 surface, 

 there are at least two layers; and the bark coating the sub- 

 stance of a shoot, besides being itself compound, includes 



