88 MEANING OF [CH. VI 



containing a crystal of calcium oxalate. The same 

 salt occurs scattered in the soft phloem, but here the 

 crystals are more complex and have a star-like radiate 

 form, as shown in fig. 40. 



The hard phloem is what gives the tough resisting 

 character to the bark of trees, and what in the lime tree 

 yields the strong rope-like material known as bast. 



In oak bark there is another hard resisting tissue 

 shown in figs. 39, 40. The tissue is made up of rounded 

 cells with small cavities and thick ligoified walls of great 

 hardness, belonging to the type known as sclerenchymatous. 

 The sclerenchyma of the oak is easily recognised by tlw 

 numerous deep narrow simple pits which traverse the 

 cell walls. 



Physiology. 



It is not at first obvious why plants should have 

 developed into such huge structures as many trees are. 

 Why should there be Sequoias in America and gum trees 

 in Australia towering two or three hundred feet into the 

 air ? This question is asked from an evolutionary point of 

 view, and simply means: What advantages, connected 

 with the tree-like habit of growth, have, by means of 

 natural selection, guided the evolution of plants in this 

 particular direction ? The answer to such questions must 

 be highly speculative ; we can never answer them 

 dogmatically. All that can be done is to point out 

 certain undoubted advantages which a plant, in taking on 

 the arboreal habit, gains in the struggle for life. The 

 chief gain is no doubt that a plant, in overtopping 



