450 Outlines of Horticuliural Chemistry. 



In the centre of the wood is situated the medulla or 'pith. It 

 is a soft, celkilar, membranous substance, juicy when young, 

 and extending 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 trans- 

 ferred to those younger branches that still retain their pith in 

 perfection. 



The stem is by no means an essential part of a 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, sea- 

 son, 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, during the latter part of the summer, 

 are employed in preparing nourishment for the production of 

 the foliage and blossom in the succeeding spring; this nourish- 

 ment is perfected and deposited in the alburnum, and mixes 

 with the sap during its ascent in that season. Of a consequence 

 it is found to increase in density proportionate to the height at 

 which it is extracted. 



The Leaves are highly vascular organs, in which are per- 

 formed some of the most important functions of a plant. They 

 are very general, but not absolutely necessary, organs, since 

 the branches sometimes perform their offices ; such plants, 

 however, as naturally possess them, are destroyed, or greatly 

 injured, by being deprived of them. The duration of a leaf 

 is, in general, but for a year, though in some trees, &c., they 

 survive for twice or thrice that period. These organs are, in 

 general, of a green colour. Light seems to have a powerful 

 influence in causing this ; since, if kept in the dark, they become 

 of a pale yellow, or even white hue, unless uncombined hydro- 

 gen is present, in which case they retain their verdure, though 

 light is absent. Hence their etiolation would seem to arise 

 from being unable to obtain, under ordinary circumstances, 

 this gas, except when light is present. Now, the only source 

 from which they can obtain hydrogen, is by decomposing 

 water; and how light assists in the decomposition may, per- 

 h aps, be explained by the disoxygenising power with which it 



