DIV. ii PHYSIOLOGY 245 



tions usually cease. Since oxygen is also essential to the human 

 organism, this fact does not seem surprising (cf. p. 273). 



Carbon Dioxide. It appears at first sight much less self-evident 

 that carbon dioxide should be indispensable to the plant, and yet 

 this is the case. While no source of carbon is offered to the plant 

 in a water culture, it grows in the food-solution, and accumulates 

 carbon in the organic compounds of which it consists ; the only 

 possible conclusion is that the plant has utilised the carbon dioxide of 

 the atmosphere. Carbon dioxide is present in ordinary air in the pro- 

 portion of 0'03 per cent. If such air is passed over a green plant 

 exposed to bright light, it can be shown that the carbon dioxide 

 diminishes in amount or disappears. Colourless parts of the plant, or 

 organisms like tjie fungi which are not green, behave differently ; they 

 absorb no carbon dioxide. If a green plant is placed in a bell-jar and 

 supplied with air freed from carbon dioxide, its growth soon stops, and 

 increase in dry weight ceases completely. Carbon dioxide is thus an 

 indispensable food-material, and is evidently the source from which the 

 plant obtains its carbon. The small proportion of this gas present in 

 the atmosphere is quite sufficient for the nutrition of plants (p. 251). 

 A supply of. organic compounds of carbon in the soil or culture solution 

 does not enable a plant to dispense with the carbon dioxide of the air ; 

 in any case CO., is the best source of carbon for the green plant which 

 we are at present considering. Neither is it sufficient to supply such 

 a plant with carbonic acid in the soil or culture solution ; it requires 

 to be supplied directly to the leaves. 



Other Gases. Oxygen and carbon dioxide are the only gases 

 which are necessary to the plant. For most plants the nitrogen of 

 the atmosphere is of no use (cf. p. 259). 



Absorption of Gases. Carbon dioxide and oxygen in part enter 

 the epidermal cells, and partly pass by way of the stomata into the 

 intercellular spaces, from which they reach the more internal tissues. 



There are no air-filled canals or spaces in the cell wall or the 

 protoplasm through which gases could diffuse into the cell. Thus 

 absorption of gases is only possible in so far as they are soluble in 

 the water permeating the protoplasm and wall. The gases behave 

 like other dissolved substances and diffuse into the cell. They 

 diffuse through cell walls more easily the richer in water these are. 

 The ordinary cell wall, when in a dry condition, hardly allows gases 

 to diffuse through it ( 28 ) ; in nature, however, the cell wall is always 

 more or less saturated with water. The cuticle, on the other hand, has 

 very little power of imbibing water, and places considerable difficulty 

 in the way of any diosmotic passage of gases ; it is not, however, com- 

 pletely impermeable. 



The gaseous diffusion takes place rather through the substances 

 with which the cell wall is impregnated than through the substance 

 of the wall itself. Since carbon dioxide is much more readily 



