COURSE OF THE SAP. 45 



bud of any tree (Fig. X. e.) is compared with the bulbs of the 

 Tiger Lily (Fig. X. A.) 



Since leaf-buds are thus the parents of wood, one of the 

 means of propagating the individual to which they belong, the 

 origin of branches, and consequently the source of the develop- 

 ment of leaves themselves, they may be considered the most 

 important organs of vegetation, so far as any one organ can be 

 called most important where aU are so mutually dependent the 

 one on the other, and so powerfully concur in maintaining the 

 system of vegetable life, that it is difficult to abstract one part 

 without impairing the efficiency of the remainder. 



The office of the stem is, to convey the crude fluid obtained 

 by the roots from the soil, and called sap, into the leaves for 

 elaboration, and then to receive it back again. Sap is, 

 originially, water holding in solution gaseous matter, especially 

 carbonic acid, together with certain earths and salts, but as 

 soon as it enters the stem, it dissolves the vegeta,ble mucilage ■ 

 it finds there, and becomes denser than it was before; it is 

 further changed by the decomposition of a part of its water, 

 acquires a saccharine character, and, rising upwards through 

 the whole mass of wood, and more especially the alburnum, 

 takes up any soluble matter it passes among. Its specific 

 gravity keeps thus increasing till it reaches the summit of the 

 branches ; by degrees, it is wholly distributed among the 

 leaves. In the leaves it is altered, and then returned into the 

 general system, more especially into the fruit, and the bark, 

 through which it falls, passing off horizontally through the 

 medullary rays into the interior of the stem, and fixing itself 

 in the interior of the bark, especially of the root, when it 

 undergoes various changes, the results of which are known 

 under the name of vegetable secretions. 



It may be said, that, in trees, the alburnum and liber have 

 each two equally important offices to perform : the alburnum 

 giving strength and solidity to the stem, and chiefly conveying 

 sap upwards ; the liber not only conveying sap downwards, but 

 covering over the alburnum, protecting it from the air, and 

 enabling it to form without interruption. The central wood is 

 of little consequence, and may be destroyed, as it constantly is 



