438 MANUAL OF BOTANY 



fungi which infest the cortical tissues and develop peculiar 

 tubercular structures upon the roots. The actual mode of absorp- 

 tion in this cise also is obscure ; the parts played by the root 

 and the fungus respectively are not at all determined. 



The water taken up is the main source of the hydrogen and 

 oxygen which are used in the anabolic processes. A little of 

 both these gases is taken in in the several combinations of the 

 metals ; sulphates and phosphates contain both, nitrates and 

 cirbonates contain oxygen. The amount of them absorbed m 

 these forms is, however, relatively small. The other elements 

 mentioned enter the plant in various combinations in the water 

 stream. 



The gases present in solution in the water of the soil also 

 make their way into the root -hairs with the stream, but the 

 quantity is very slight compared with what is absorbed by the 

 aerial parts. The gas CO.,, wliich we have seen to be present in 

 the earth in considerable amount, is, however, not made use of 

 in the anabolic processes. All of this particular food material is 

 taken in from the air. A little carbon is absorbed in the form 

 of carbonates. More complex organic compounds of carbon are 

 taken in by those roots with which fungi are living symbiotically, 

 such as the mycorhiza of some trees, but this is exceptional. 



The absorption of gases from the air takes place in the 

 leaves and other green parts. They enter freely through the 

 stomata and find their way thus into the intercellular spaces, 

 which are very numerous. These intercellular spaces contain 

 a mixture of gases which, though approximating to the com- 

 position of atmospheric air, yet differs from it in the relative 

 quantities of the constituents. This can be readily understood 

 from the consideration that considerable gaseous interchange 

 goes on in these spaces between the air they contain and the 

 living cells which abut upon the spaces. The cell-walls here 

 are very delicate and thin, and are saturated with water. In 

 this water the different gases present dissolve according to their 

 degree of solubility. The quantity of each taken up depends, as 

 in the case of the metallic salts just discussed, upon the ability 

 of the cells to make use of the gas. If it can be combined in 

 any way with other bodies in the cell, it is withdrawn thus from 

 the sap and room made for more to enter. If not, the limit of 

 saturation of the sap is soon reached. 



Probably little or no absorption of gas takes place through 

 the cells of the epidermis, though they are freely in contact with 

 the air. Sections show us that they are provided usually with a 



