SECT. 11 niYSIOLOGY 211 



water, in which case they are absorbed in the same way as other 

 dissolved substances. 



Oxygen. — When a plant is deprived of oxygen, either by exhausting 

 the air or by cultivation in an atmosphere of carbon dioxide, nitrogen 

 or hydrogen growth usually ceases at once. Since oxygen is also 

 essential to the human organism, this fact does not seem surprising 

 (cf. also p. 241). 



Carbon Dioxide. — It appears at first sight much less plausible to 

 the laity 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"0.3 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 the 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 

 ultimately other injurious effects may be manifested ; in any case in- 

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

 indispensable food-material, and is evidently the soui'ce 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. 217). 

 A supplj^ 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. 



In any attempt to distinguish the relative importance of substances 

 utilised in plant nutrition, carbon undoubtedly ranks first. Every 

 organic substance contains carbon, and there is no other element 

 which could supply or take part in the formation of so many or 

 such a variety of substances, in living organisms as in the chemical 

 laboratory. Organic chemistry, in short, is merely the chemistry of 

 carbon compounds. Living l^eings which are composed of organic 

 substances owe the possibility of their existence primarily to the 

 properties of carbon. 



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. 



Absorption of Gases. — Carbon dioxide and oxygen in part enter 

 the epidermal cells, and parti}' pass by way of the stomata into the 

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



