134 SOUTH AFRICAN BOTANY 
2 and 8, in fact there will be almost no loss in the one 
placed in the dark. 
Another method of showing that sunshine and wind 
affect transpiration is by using a piece of apparatus 
called a potometer. A description of this will be found 
in the Appendix. 
Arrangements for reducing the rate of transpiration 
found in many plants, are discussed later, under the 
heading Xerophytes. 
102. Carbon Assimilation.— We have seen that car- 
bon is found in all green plants, but is not absorbed by 
the roots. If the plants grown by water culture are 
tested, they will be found to contain a considerable 
amount of carbon, yet none was supplied in the solution. 
It is hence obvious that they must obtain it from the 
air. There is in the air a small quantity of a certain 
gas called carbon di-oxide, which is a compound of car- 
bon and oxygen. It is produced by the breathing of 
animals and plants, and by all processes of burning, if 
the substance burnt contained carbon, e.g. burning coal 
or wood. It is an invisible gas, but it has the property 
of turning lime-water milky, hence its presence can be 
demonstrated by leaving a dish of lime-water exposed 
to air for some time, when a scum forms on the surface. 
The leaves absorb this carbon di-oxide; they break it 
up into its elements, carbon and oxygen; they keep the 
carbon and give out the oxygen. The carbon, together 
with water absorbed by the roots, is made up into cer- 
tain complex substances, chiefly starch. This process 
of forming organic compounds from carbon di-oxide 
and water is known as CARBON ASSIMILATION or PHoTO- 
