THE STEM AND BRANCHES.- I9l 



hating protoplasm, to guide their ffioVements. 

 Whence do they get it ? From the factory in 

 the foliage. Thus, from the summit of the tallest 

 tree down to the lowest root that fastens it in 

 the soil, there runs a complex system of pipes 

 and tubes for the special conveyance of elaborated 

 material ; and this system supplies every grow- 

 ing part with the food- stuff necessary for its 

 particular growth, and every living part with the 

 food-stuff necessary for maintaining its life and 

 activity, An interchange of protoplasmic matter, 

 starches, and sugars, goes on continually through 

 the entire organism. 



This downwar^^d fmf.wa.rrl stream of living \ / 

 matter, carrying along with it live protoplasm 

 and other foods or manufactured materials, must 

 be carefully distinguished from the upward 

 stream of crude sap which rises from the roots ' 

 to the leaves and branches. The one contains 

 only such raw materials of life as are supplied 

 by the soil namely, nitrogenous matter, water, 

 and food-salts t ; the other contains the things 

 eaten from the air by the plant in its leaves, I 

 and afterwards worked up by it into sugars,/ 

 starches, protoplasm, and chlorophyll. 



Stems are usually covered outside for purposes 

 of protection by a more or less thick integument, 

 which in trees and shrubs assumes the corky 

 form we know as bark. Bark consists of d,eacl 

 and empty cells, thickened with a lighter 

 thickening matter than wood, and presenting 

 as a rule a rather spongy appearance. But 

 beneath the bark comes a ^i^nci^_layer_Qf living 



