CH. III.] THE STEM AND BRANCHES. 138 



trees in clumps and avenues ; their external sides 

 will be found to enlarge much more rapidly than 

 their internal or most shaded sides. 



In the centre of the wood is situated the medulla 

 or pith. It is a soft, cellular, membranous substance, 

 juicy when young, and extendmg from the ends of the 

 roots to the extremities of the branches. In the first 

 stages of vegetation, it occupies but a small space : it 

 gradually dilates ; and in shoots of a year old, and in 

 young trees, it is of considerable diameter; as their 

 age increases, it gradually diminishes and at length 

 becomes totally extinct, its place being occupied by 

 perfect wood. Its functions are little understood. 

 It appears to be connected with the production of 

 young shoots ; for, as soon as it becomes extinct in 

 a branch, that member loses, in a great degree, the 

 power of producing them ; that power apparently 

 being transferred to those younger branches which 

 still retain their pith in perfection. 



The stem is by no means an essential part of the 

 plant, since many are destitute of it ; to such trees as 

 naturally are gifted with one, it is somewhat injurious 

 to prevent its formation. Standard fruit trees, under 

 similar circumstances of soil, season, and culture, 

 generally, produce finer flavoured fruit than either 

 dwarf standards or espaliers. This fact appears to be 

 accounted for by the discoveries of the indefatigable 

 Knight, which evince that plants, duiing the latter 



