THE STEM 107 



tissues. This stream of assimilated food matter passing down 

 from cell to cell, and from sieve-tube to sieve-tube, regulates 

 the growth of the cambium — that is, the yearly increase in 

 thickness. In poor years, when the leaves are not very active, 

 or when they are not well developed {e.g., after attacks of 

 insects), it may happen that the plastic material is used up on 

 its way down, and nothing will remain for the cambium of the 

 roots, which requires it as much as that of the stem. In such 

 cases, the annual riug of growth tapers downwards, that is, 

 the annual ring will only be found so far down as the formed 

 material has been able to penetrate. In normal years, how- 

 ever, there is a surplus of food material. The tissues destined 

 for its passage are not sufficient for the downward current of 

 assimilated substances ; it overflows into other tissues and there 

 deposits the superfluous matter. In this case, the parenchy- 

 matous cells of the cortex, of, the medullary rays, and of the 

 pith become filled with starch, which remains in reserve for 

 future use. 



If trees are left to themselves, their upright trunk becomes 

 more imposing by the death and decay of the lower branches. 

 Leaving out of account the damage of the branches by herbi- 

 vorous animals, the tree is able to rid itself of its superfluous 

 branches. Either through the overshadowing of its neighbours, 

 or through that of its own upper branches, the more delicate 

 lower branches die off and are torn away by the wind. 



The formation of a trunk is therefore advantageous to a 

 tree, and called forth by the natural condition of growth and 

 of its surroundings. In cultivated plants, however, it is not 

 always advantageous nor necessary. This has brought about 

 the cultivation of dwarf-trees. 



