ABSORPTION OF CO, 439 



cuticle, which resists an}^ such sohition as is necessary for 

 absorption. 



The only gas which is absorbed as a food material from the 

 air is carbon dioxide, CO ,. This exists to a very slight extent in 

 the atmosphere, only about four parts in ten thousand being 

 normally present. The very large green surface which an 

 ordinary terrestrial plant possesses renders, however, a consider- 

 able amount of absorption possible. If the general conditions 

 are favourable, the absorption is continuous, for CO., is at once 

 decomposed in the cells of the green tissue, and so a stream is 

 alwaj's entering. 



Both nitrogen and oxygen are soluble in water, though to a 

 different extent. It has been already stated that the nitrogen so 

 taken in is not used in the constructive processes, and accord- 

 ingly but little is absorbed in this way. The oxygen which 

 enters is larger in amount ; experiments have proved, however, 

 that it is not a food material, biit is used for other purposes. 



The absorption of CO., takes place usually at the ordinary 

 atmospheric pressure or at one a little greater. Plants can, 

 however, absorb this gas when it is present in much larger 

 proportion than it is in air. Too much is, however, possible, 

 and then the cells are unable to take it in at all. The absorption 

 of CO., is possible only under certain conditions ; the cells which 

 contain chloroplasts are the only ones which can take it in in any 

 quantity, and the}^ onh' when they are exposed to sunlight, prefer- 

 ably a bright light, and when the plant is maintained at an 

 appropriate temperature. Its absorption is accompanied by the 

 evolution of a volume of ox3'gen, which is equal to the volume 

 of CO., absorbed, and it is attended b^" a continuous increase 

 in the weight of the plant. 



We have seen that the water absorbed by the roots is trans- 

 ported regularly through the axis of the plant until it reaches 

 the leaves, in which after traversing the cells of the mesophyll 

 it is evaporated into the intercellular spaces. Into these cells 

 of the interior of the leaf all the food materials are thus at 

 once transported, both those entering from the soil and those 

 absorbed fi^om the air. These mesophyll cells have generally a 

 different arrangement on the two sides of the leaf, but they all 

 agree in containing chloroplasts. In these cells takes plaoe the 

 work of construction of organic substance, such as the plant can 

 live upon, work which is carried out mainly through the instru- 

 mentalit}' of the chloroplasts. 



As already noticed, various elements are constantly found in 



