FOOD OBTAINED FROM THE AIR 35 
a thin film of carbonate of lime will begin to cover the 
surface of the liquid. In this case the carbon-dioxide 
combining with the lime was supplied by the air. The 
following considerations will help to reinforce what these 
experiments teach. | 
In making culture solutions which we shall have occasion 
to refer to further on, it is unnecessary to include the 
element carbon. These culture solutions consist of mineral 
salts which replace the food a plant ordinarily obtains from 
the soil, so that evidently a plant does not rely on the soil for 
its supply of carbon. We know, however, that carbon in the 
form of carbon-dioxide exists in the air, and we have seen 
moreover that the air is able to enter the plant through its 
leaves ; it seems a reasonable inference that the air supplies 
the plant with carbon. An experiment will enable us to 
confirm this surmise. 
One of the compounds containing carbon which are formed 
by plants is starch, and starch is easily recognised ; for it gives 
a characteristic blue colour when treated with a solution of 
iodine (see page 3). Take a green leaf which has been exposed 
to sunlight and boil it for a few minutes in water, next 
immerse it in alcohol fora short time and its green colouring 
matter will be dissolved out. Having washed it well 
in water to remove all traces of the green solution, 
place it in a weak solution of iodine (in which a small 
quantity of potassium iodide has been dissolved) and at 
once the bleached leaf will assume a blue or purplish brown 
colour showing the presence of starch. This shows that 
carbon has entered the leaf when it is exposed to ordinary 
air. 
We have yet to show that a leaf, if exposed to air from 
which the carbon-dioxide has been withdrawn, cannot form 
starch—that it is unable to obtain the necessary carbon. 
The following experiment will make this clear. Fit up a jar 
