GLIMPSES INTO PLANT STRUCTURE 45 



plant or tree, and in the growing season is con- 

 stantly forming new cells by division. By this 

 means new vessels and wood-cells are formed. 

 And as the new cells keep multiplying they 

 exert pressure upon their surroundings, so that 

 the tree gradually expands and increases in 

 girth during the growing season. In the autumn 

 a stronof outer coat of bark is formed to 

 enclose and protect the living cells. Dormant, 

 yet full of life, these remain throughout the 

 cold and wintry weather ; but in the spring 

 active life begins again. The cells commence 

 once more to divide and multiply, and the outer 

 coat of bark, which was firm and strong when 

 it enclosed its living successors — for it origi- 

 nates from these soft, active cells — is burst 

 asunder as the sap rushes through the cells and 

 distends them. Thus the rough broken bark, 

 with which we are all familiar, is formed upon the 

 trunks of trees. 



During the spring and summer seasons the 

 growth and multiplication of the cells goes on, 

 but pauses again in winter ; and in due course, 

 when the woodman comes along, he can approxi- 

 mately estimate the age of the tree by its annual 

 rings of growth. The successive zones of growth 

 are usually quite distinct, owing to the wood 

 formed in the spring being less dense or having 



