58 NUTRITION. 



perishes in a longer or shorter time according 

 to circumstances. If the ascending current were 

 impeded, it is obvious the accumulation which 

 causes the tumour, must take place on, or below 

 the lower lip of the incision. This descending 

 sap, or proper juice, whose chemical composi- 

 tion appears to be water and carbon, and which 

 itself principally in the form of gum, is capable 

 of being, by very slight modifications, trans- 

 formed into fecula (starch), sugar, and lignine, 

 quits the leaves during the night, and traversing 

 the bark and pith in exogenous, and the wood 

 in endogenous plants, reaches the roots. In its 

 progress it deposits nutritious matter, which, 

 more or less mixed in the woody portions with 

 the ascending sap, or absorbed with the water 

 which is taken up through the medullary rays 

 by the cellular envelope, is imbibed by and elab- 

 orated in the cells. It meets in its course and 

 especially in the bark, glands and glandular 

 cells, which imbibe it and form in their cavities 

 peculiar secretions (51) most of them incapable 

 of nourishing the plant, and destined to be re- 

 jected or carried into the substance of the 

 tissue. 



The water which rises from the roots to the 

 foliage is almost as pure when it reaches it, as 



