26 TYPES OF BRITISH PLANTS 
important work. They seize on the carbonic acid, which 
comes in through the mouths, pores, or stomata of the 
skin, and break it up into carbon and oxygen. The 
oxygen they do not want for the time, as they can get 
sufficient from the water in the cell, so that is expelled 
from the leaf, but the carbon they keep, and combine 
with the hydrogen and oxygen of the water into starch 
or sugar. With the microscope one can see the starch 
grains forming in the chlorophyll. As you will re- 
member, starch and sugar are alternative forms of 
cellulose, of which the cell-walls are made, and so from 
the leaves in one form or another this raw material is 
taken round to places where cells are splitting up fast, 
as for instance to the cambium ring inside the trunk. It 
is just as necessary where cells are not splitting into 
more, but merely growing lengthwise or having their 
walls thickened. In any case they want bricks for the 
wall, and the protoplasm gets the raw material from the 
leaves, changes it into cellulose, and then as it runs 
around the walls of its cells keeps putting little particles 
in, and either stretching or thickening the membrane, or, 
if the cell be dividing, building up a fresh party wall. 
Very often the starch grains are not used up at once, but 
are taken and stored in suitable places, as in the sheath 
around the bundle, in tubers like the potato, or in seeds, 
ready for next year’s growth. Thanks to these stores, a 
potato in a cellar, without any soil, can go on merrily for 
quite a long time sending out a shoot, and building up 
the stalks as it goes along with cellulose taken from the 
reserve laid up. 
Cell-walls, however, would be useless by themselves, 
and the living protoplasm must be nourished too. Besides 
the solid carbon, and the two gases, oxygen and hydrogen, 
