' ASSIMILATION,' OR ' PHOTOSYNTHESIS ' 209 



air is very small, averaging about 2 '8 parts in. 10,000, it is from 

 this source that the whole of the carbon of plants grown by the 

 method of water-culture is derived. 



In the processes of fermentation and decay going on in 

 ordinary soil carbon dioxide is produced and the air permeating 

 the interstices of the soil may contain as much as 5 per cent, of 

 this gas, some of which enters the roots of plants dissolved in 

 the water of the transpiration-current : it has, however, been 

 shown by Cailletet and Moll's experiments that the supply 

 of carbon dioxide obtained in this manner is insufficient for the 

 requirements of ordinary green plants. 



Extended and carefully-conducted investigations have proved 

 beyond doubt that the chief food-material utilized by green 

 plants for their carbon-supply, is the carbon dioxide of the air, 

 and that this gas is absorbed by means of the leaves. Moreover, 

 it is through the stomata that the gas enters into the tissues 

 and only in slight degree, if at all, through the cuticle of 

 the epidermal cells. 



The rate at which the absorption of the gas is carried on by 

 the leaves has been recently investigated by Brown and Escombe : 

 the amount absorbed by a sunflower exposed to diffuse daylight 

 was found, in one instance, to be 412 cubic centimetres per 

 square metre of leaf-surface per hour ; the hourly absorption for 

 a Catalpa leaf was 345 c.c. for each square metre. Under 

 favourable conditions the rate of absorption of the gas by a leaf 

 was found to be equal to one-half that of a strong solution of 

 caustic potash of equal area, and, since the actual openings 

 between the guard-cells of the stomata in the leaf investigated 

 amounted to not more than -j-^ part of the whole area of the 

 leaf, it follows that the rate at which carbon dioxide entered 

 was fifty times as rapid as that at which the gas is absorbed by a 

 solution of caustic potash, a truly astonishing result. 



This absorptive activity on the part of green vegetation would 

 soon result in the total removal of carbon dioxide from the air, 



o 



