32 VITAL ACTIOHS. 



to abstract one part without impairing the efficiency 

 of the remainder. 



56. 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 re- 

 ceive it back again. Sap is, originally, water contain- 

 ing various gases, earths, and salts, in solution : 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 decompo- 

 sition of a part of its water, acquires a saccharine 

 character, and, rising upwards through the alburnum, 

 takes up any soluble matter it passes through. Its 

 specific gravity keeps thus ^increasing till it reaches 

 the summit of the branches; and, by degrees, it is 

 all distributed among the leaves. In the leaves it is 

 altered, and then returned into the stem ; not, how- 

 ever, into the alburnum, where it would meet the as- 

 cending current, but into 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. 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 conveying sap upwards ; the liber not only con- 

 veying sap downwards, but covering over the albur- 

 num, protecting it from the air, and enabling it to 

 form without interruption. It is, therefore, indispen- 

 sable to the healthy condition of plants, that neither 

 the alburnum nor the liber should be injured. The 



