■^'THE STEM 105 



peeling off of the bark scales. This causes the pressure within 

 the stem to be greater, the aeration of the cortex is reduced, 

 and the thickening of the stem is kept back. In such cases 

 the bark will suffer considerable decay, and a large develop- 

 ment of lichens will take place on the trunk of the tree. These 

 stems covered with moss and lichens are generally due to an 

 excessive external pressure of the cortex, and should warn us 

 to take some measure against it. These consist in scraping 

 the stem or making longitudinal incisions as well. 



The practice, which is gaining ground, of scraping' the stems 

 of our cultivated trees, is very much to be recommended as a 

 means of preventing the excessive pressure of the cortex, and 

 has the secondary advantage of preventing the old bark from 

 harbouring insects and their eggs. 



§ 18. How does the stem as a whole perform its functions ? 



We have sketched out in the preceding paragraphs the 

 structure, arrangement, and functioning of the wood and bast 

 systems, and of the cambium ; it is now necessary to add a 

 few words about the central cylinder of the pith. This latter 

 consists of parenchymatous cells, which in a few families of 

 plants is traversed by strands of soft bast, or even by entire 

 vascular bundles — that is, by tissues meant for the conduction 

 of formed substances. Such medullary phloem bundles are 

 present, for instance, in the tuberous Begonias, which have to 

 fill their storage tubers with reserve material for the leafless 

 period of rest; they are absent in the ordinary Begonias. 

 Medullary bundles exist, therefore, in plants which withdraw 

 their reserve substances into the roots. From the fact that in 

 many plants the pith may die off, split, dry up, and disappear, 

 without affecting the stem, we may gather that the chief func- 

 tion of the pith must occur during the early development of 

 the stem. As a matter of fact, experiments go to prove that 

 the function of the pith is to swell up the axis, and therefore 

 to accelerate the growth in length of the stem. If we cut a 

 piece pf a young stem or branch of the Elder (Samhuctis), and 

 place it in water, we see after a short time that the pith 

 ijulges out from the cut end, which indicates that by taking 



