52 THE LIFE OF THE PLANT 



modified chemically. This is nothing but collodion, 

 such as is used by photographers. The bladder (A) is 

 joined to a horizontal glass tube (B) which contains a 

 drop of a coloured liquid (a). We can judge whether 

 the volume of air in the bladder increases or decreases 

 according to the movement of the drop towards or from 

 the bladder. I let the bladder down into the broad 

 and empty vessel (C) and pour into it some carbonic 

 acid. You cannot see it because carbonic acid is a gas 

 as colourless as air. But I am correct in saying that I 

 pour in the carbonic acid because it is heavier than air, 

 and so can be poured from one vessel into another 

 while remaining totally invisible. Carbonic acid can 

 be kept for a certain length of time in a vessel open at 

 the top in the same way as hydrogen can be kept for a 

 short time in a bell open at the bottom. After having 

 introduced carbonic acid into the vessel surrounding the 

 bladder, the drop of coloured liquid trembles and then 

 runs along in the direction of the arrow, thus demonstrat- 

 ing that carbonic acid has begun to penetrate through 

 the moist wall of the bladder, which is our artificial cell, 

 and moreover that it is doing so more quickly than the 

 air is escaping from the bladder. In the same way a 

 vegetable cell has no need to attract or imbibe gases, 

 such as carbonic acid, which, owing to its property of 

 diffusion, will penetrate independently any cell devoid 

 of it. 



Now let us observe the behaviour of vegetable cells 

 towards substances dissolved in the water of the soil. 

 Let us take some oblong bladders made of the same 

 collodion and fix them to the extremities of lamp 

 glasses (fig. 19). Suppose these collodion bladders 

 represent root-cells, by means of which the plant comes 

 into contact with the nutrient substances contained in 

 the soil. A plant, as we know from its chemical com- 

 position, needs among other things iron salts. We 

 choose these for our illustration, because they give 



