696 PART IV. — THE PHYSIOLOGY OF PLANTS. 



after absorption. For instance, plants absorb from the air 

 nitrogen, oxygen, and carbon dioxide under certain circarastances. 

 On account of the greater proportion of nitrogen in the air, this 

 gas will be, in the first instance, absorbed to a larger extent than 

 either of the others. But inasmuch as the nitrogen does not enter 

 into the metabolic processes of the plant, whereas oxygen and, 

 under certain circumstances, carbon dioxide, do so, the nitrogen 

 simply accumulates in the cell-sap, and the absorption of it will 

 soon cease, whereas the absorption of the oxygen and of the carbon 

 dioxide will continue. 



Land-plants absorb gases, in the manner described above, at all 

 points of their surface ; by their shoots from the air, by their roots 

 from the gaseous mixture in the interstices of the soil ; the stomata 

 of the sub-aerial parts are of great importance in connexion with 

 this process. Submerged water-plants absorb, in solution, the 

 gases dissolved in the water. 



The absorbed gases remain in solution in the cell-sap, so that 

 living cells do not contain babbles of gases. Moreover, gases 

 travel in the plant mainly by diffusion from cell to cell, though 

 their distribution may also be effected by means of the intercellular 

 spaces. 



The particular gas which may be regarded as a food-material is 

 carbon dioxide. Although the amount of this gas to be found in 

 the air, or in solution in water, at any given moment, is usually 

 small, yet inasmuch as the supply is maintained, enough of it can 

 be obtained for the needs of the plant. The conditions of its 

 absorption are the following : First, it is only absorbed by those 

 cells which contain chlorophyll ; Secondly, it is only absorbed by 

 these cells when exposed to light of a certain intensity, the most 

 active rays being those of low refrangibility. 



§ 7. Transpiration. Every part of a plant which is exposed 

 to the air, except such as are covered by a thick layer of cork, is 

 continually exhaling watery vapour. This may be demonstrated 

 by placing a leafy branch under a bell-glass, when it will shortly 

 be observed that the internal surface becomes covered with drops 

 of water, the watery vapour exhaled by the branch having con- 

 densed upon the cold glass. Again, the drooping of cut flowers 

 or herbaceous branches is due to the loss of water by ti'anspira- 

 tion. 



It must be clearly understood that transpiration is not simply 

 evaporation. If it were so, then clearly equal amounts of water 



