THE STEM AND BRANCHES. 191 



nating protoplasm, to guide their movements. 

 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 downward and outward 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 ; the other contains the things 

 eaten from the air by the plant in its leaves, 

 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 dead 

 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 distinct layer of living 



