CHAPTER II. GASES IN THE PLANT- 233 



Carbon dioxide, which, as we have seen, is of extreme import- 

 ance to the plant as food, being the chief if not the only source 

 of the carbon which forms nearly one-half of the dry weight of 

 every plant, is absorbed not only from the air directly by the 

 green cells in the assimilative process, but some finds its way to 

 the interior of the plant in solution in the water absorbed by the 

 roots. It is also produced in the process of respiration, and 

 given off as waste matter by all the living cells throughout the 

 body of the plant. 



Oxygen is also obtained by the plant from the air direct, and 

 is taken up in solution by the roots, but it is also set free in large 

 quantities by the assimilative processes in the green parts of 

 the plant. 



Nitrogen, which forms a normal constituent of the air, is 

 found both free and in solution in all parts of the plant, being 

 absorbed along with other components of the air, both directly 

 and in aqueous solution through the roots, but it does not appear 

 to play any important part in the vital processes of the plant. 



Watery vapor exists in the cavities of the plant in larger or 

 smaller proportion, and is exhaled in abundance from the 

 stomata. 



In the living plant these gases are never absolutely quiescent, 

 or in a state of perfect equilibrium. Owing to the chemical 

 activity of the cells, to the evaporation constantly going on at 

 the surface, to the varying temperature of the air, causing expan- 

 sion or contraction of the gaseous contents of the plant, and to 

 the mechanical agitation caused by the wind moving among the 

 leaves and branches, the gases in the plant are in constant move- 

 ment, though the movements are by no means regular or 

 uniform. 



One interesting effect due to the presence of gases in the 

 plant is seen in the bleeding of trees when wounded in early 

 spring. At this season of the year, before the buds begin to 

 unfold, and there are few stomata or other escapes for the pent 

 up gases, and when, at the same time the tissues are gorged with 

 sap, if the tree be wounded, the expansion of the gases on the 

 interior, caused by the heat of the day, forces out a portion of 

 the sap. In some trees, as the Maple, the flow is so copious and 

 so sweet that it is profitably employed as a source of sugar. 



