136 PROCESS OF NUTRITION. CHAP. III. 



of bleeding; the sap that issues from the wound 

 possesses properties very different from the mere 

 moisture of the soil, as is indicated by means of 

 chemical analysis, and sometimes also by means 

 of a peculiar taste or flavour, as in the case of the 

 Birch-tree. Hence the sap has already undergone 

 a certain degree of elaboration ; either in passing 

 through the glands of the cellular tissue, which it 

 reaches through the medium of a lateral commu- 

 nication, or in mingling with the juices contained 

 in the cells, and thus carrying off a portion of them ; 

 in the same manner, we may suppose, that water by 

 filtering through a mineral vein becomes impreg- 

 nated with the mineral through which it passes. 



But this primary and incipient stage of the 

 process of elaboration must always of necessity 

 remain a mystery to the phytologist, as being 

 wholly effected in the interior of the plant, and 

 consequently beyond the reach of observation. 

 All he can do, therefore, is to trace out its future 

 progress, and to watch its succeeding changes, in 

 which the rationale of the process of elaboration 

 may be more evident. 



But is The next, and indeed the principal, part of the 



fectecHn process of the elaboration of the sap is operated 



* n ^ e ^ ea ^ : * r t ^ le sa P no sooner reaches the leaf 

 than part of it is immediately carried off by means of 

 perspiration, perceptible or imperceptible ; effecting 

 a change in the proportion of its component parts, 

 and by consequence a change in its properties. 



