PHENOMENA CONNECTED WITH GROWTH. 21 



tissues are filled in they become hard, and of a 

 deeper colour. This is the "heart" of the tree, 

 but the sap does not pass through these obstructed 

 vessels any more. This portion, however, is the 

 real support of the tree against violent winds. In 

 like manner each year sees new liber grow, while 

 the oldest layers, now external, possess no longer 

 vitality, and become dry and wrinkled. 



The growth of roots is owing to the descent of 

 the cambium, by the addition of new cellular tissue 

 to the spongioles. It is evident, then, how muti- 

 lation of these is injurious to the welfare of the 

 tree, and how a healthy equilibrium between root 

 and shoot is a first requisite to its healthy vitality. 

 When these spongioles become obstructed they 

 cannot absorb the materials of the soil, and thus 

 the tree languishes and dies. A portion of the 

 cambium in the autumn does not descend to the 

 roots, but is reserved by nature to feed the young 

 shoots before the leaves expand in spring. In 

 shoots, the inner epidermis is formed by the aid 

 of this reserved cambium. The first action of the 

 spongioles in spring may also be due to its in- 

 fluence, in some measure. 



In like manner we may readily understand the 

 growth of the fruit. Fruit is composed of cellular 

 tissue ; like the leaves, the epidermis is full of 

 stomata or mouths. The parenchyma of fruits has 



