THE PARTS OF A PLANT AND THEIR WORK 19 
found to have their guard-cells so arranged that the spaces 
between them permit water to evaporate readily; but if ex- 
amined when the plant is too dry, they will be found closed, 
or nearly so, thus tending to prevent loss of the scanty 
supply of water. 
While the root-hairs are taking in water and sub- 
stances in solution, and while these are being transported 
to the leaves, a gas called carbon dioxid is taken from the 
air into the interior of the leaf. However, neither water 
and the substances it has in solution, nor the carbon dioxid 
can be used directly as nourishment, but must be made first 
into other substances. 
The water and the carbon dioxid are made into starch 
or sugars or other things similar to these, which can be 
used as food for the plant. While they are being made a 
surplus of oxygen appears as a waste. Some of this may 
be used by the plant, but most of it escapes from the leaves, 
and is used by other plants and the animals. 
The process of making starch, sugars, and similar sub- 
stances requires a large amount of energy, and this is ob- 
tained by means of the chlorophyll as the sun’s rays fall 
upon it. Chlorophyll is said to absorb energy from sun- 
light. Only those plants which have chlorophyll can obtain 
the needed energy in this way, therefore they only can 
manufacture these food materials and set free oxygen. All 
animals obtain their food directly or indirectly from plants, 
and since plants are themselves dependent for food upon the 
agency of chlorophyll and sunlight, it follows that all plants 
. and animals are dependent upon chlorophyll and sunlight.* 
The substances from the soil that are carried up in solu- 
tion in the water are not used in making starches, sugars, 
etc., but are afterward added to them in the formation of 
proteids and other foods. 
1 Tt has been found that an intense artificial light, as electric light, 
falling upon chlorophyll may also enable plants to manufacture food, 
