60 Fruit-growing in Arid Regions 



of bark, when dead, are split by pressure from within, and 

 gradually drop off. 



We can now understand how the trunks of all tree fruits 

 with which our orchardists have to deal increase in size by 

 the growth of a ring of wood each year just beneath the 

 bark. By counting the rings, one may determine the age 

 of a tree with a fair degree of accuracy. As the tree grows 

 older, the inner wood becomes inactive, and so takes but 

 little part in the life processes. This accounts for the fact 

 that a hollow tree a mere shell of a trunk may live 

 as long as though it were perfectly sound, if the tree does 

 not break down. 



It is a common notion that the split made in limbs in 

 grafting will grow together, but we can now see that this is 

 impossible and, moreover, that the region where a union 

 of the stock and cion can take place is very small. This 

 necessitates careful handling of the parts to prevent injury, 

 as well as to see that the cambium layers of cion and stock 

 come into intimate contact. 



The movement of sap up the stem and the channels 

 may be illustrated by inserting a freshly cut succulent 

 stem in red ink. After a time it will be found that the 

 ink has ascended the stem for some distance, and the 

 water vessels above described are. the only parts which are 

 colored. 



This function of the vessels of sapwood is also proved by 

 the action of girdled trees. If a ring of bark only is re- 

 moved without injuring the wood, a tree may live through 

 an entire season; but if a ring of sapwood is removed at 

 the same time, the tree will soon die. 



Girdling or ringing a tree will also illustrate where the 



