8 LINKS WITH THE PAST [CH. 



Red Sea which are still tenanted by living polyps 

 may have been growing in the days of the Pharaohs. 

 The coral polyp represents the growing portion of a 

 lifeless mass of rock which is constantly extended by 

 the activity of the organism at the summit of each 

 branch. Between a coral-reef and a tree there are 

 many essential differences, but a rough analogy may 

 be recognised. A tree, unlike the higher animals, 

 does not reach a stage at which the whole of its 

 substance attains a condition of permanence and 

 fixity. It consists of a complex branching-system 

 in which each shoot increases in length by virtue 

 of the youthful vigour of its apex : to a large extent 

 the tree as a whole consists of lifeless material in- 

 capable of further growth, as is the case of the older 

 portions of a coral-reef ; but the regular increase in 

 girth of the trunk and its branches demonstrates that 

 this comparison is only partially true, and that the 

 power of growth in a tree is not confined to the 

 extremities of the youngest shoots. The tip of every 

 twig is composed of minute cells endowed with a 

 potentiality of development like that which charac- 

 terises the embryonic tissues of a seedling just 

 emerged from the seed. In the course of its growth, 

 each branch, by means of its living and dividing- 

 cells, contributes to the several parts of the complex 

 mechanism of the tree. While the greater number 

 of cells acquire a permanent form and lose the 



