i] INTRODUCTORY 9 



power of further development, there remains a cy- 

 linder of cells endowed with perpetual youth. This 

 cylinder of living cells, known as the cambium, extends 

 between the wood and bark from one end of the tree 

 to the other : by its periodic activity it adds new layers 

 of tissue each year and thus, by increasing the amount 

 of conducting tubes for the transport of water and 

 for the distribution of elaborated food, it enables the 

 tree to respond to the increasing demands which 

 are the necessary accompaniment of increasing size. 

 It has already been pointed out that in the spring 

 when the sap flows most vigorously the cambial 

 cylinder produces larger tubes, and afterwards when 

 the tree settles down to its normal life, these are 

 succeeded by narrower and stronger tubes. These 

 later formed elements serve also an important me- 

 chanical purpose ; by the strength of their walls they 

 increase the supporting power of the tree and enable 

 it to sustain the added burden of the annual increase 

 in the weight and extent of its spreading branches. 

 It is the persistence of permanently juvenile tissue 

 in certain regions of a tree, together with the re- 

 markable power of repairing injuries and shedding 

 effete parts, that constitute some of the most striking 

 contrasts between the higher animals and plants. 

 The embryo oak in the earlier stages of development 

 consists entirely of actively growing cells ; by degrees 

 differentiation of the embryonic tissues results in the 



