132 COMPENDIUM OF GENERAL BOTANY. 



metJiods by which we are so amply siipphed with "daily bread." 

 Much less is it capable of imitating the process artificially. 



Chloroj)hyll proves to be of great importance during various 

 periods of chemical activity in plants." Among trees with decidu- 

 ous leaves we see that the assimilating organs are destroyed at the 

 close of the vegetative period. Chlorophyll itself is, however, not 

 simply lost; in the autumn before the leaves begin to fall the 

 most valuable mineral constituents (kalium, phosphoric acid) pass 

 into the enduring portions of the plant, to be again utihzed the 

 following year ; yellow grains, causing the autumn coloration of so 

 many leaves, ^ remain in the cells of the falling leaves as a waste 

 product. Chlorophyll-grains therefore undergo decomposition. 



VII. THE FUNCTION OF AERATION. 



The discussion of the fact that gas- forming and gas-requiring 

 processes take place within the cell, and the explanation of a few 

 simple observations associated therewith, will enable us to under- 

 stand correctly the structural arrangements to be discussed below. 



If one considers the fact that air never occurs in the form of 

 bubbles within the active living cell, and that the most important 

 chemical processes (assimilation of atmospheric CO, with liberation 

 of oxygen, and true respiration with liberation of CO,) take place 

 in the living cells, it is natural to conclude that the active exchange 

 of gases which takes place in the immediate vicinity of these cells ; 

 or in other words, since no gas appears in the cell in the form of 

 l)ubbles, that such gas exchange must take place hetween the cells. 

 In fact, the system of aeration of plants is intercellular, that is, it 

 is situated outside of the cell. 



The aerating system spreads labyrinth-like through the entire 

 plant-body, beginning with the vegetative point at the apex of the 

 stem and extending to the root-tip ; beginning with the pith and 

 extending radially, it crosses the wood-parenchyma, cambium, and 

 cortex. In the leaves and other organs it extends to the epidermal 

 tissue, in the form of fine canals. This system among plants living 

 in the atmosphere can be considered only as functional when there 

 are suitable anatomical arrangements to permit the ingress and 



' Sachs, Vorlesungen, p. 384. 



2 These bodies are fouud in the cell-sap of those leaves colored red in the fall. 



