4 MICROSCOPIC PLANTS. [CHAP. 
cell rapidly along. Like many other of these minute 
forms of life, Protococci are not easily destroyed. 
Shallow pools containing them may be thoroughly 
dried up by a very hot summer, yet as soon as rain 
falls the Protococci may be found there in profusion. 
When in a dry state they are often carried long 
distances by the wind, and, catching on any damp 
surface, quickly increase in numbers. Consequently, 
one has but to look in damp places to find a profu- 
sion of them. If we care to experiment a little with 
this one-celled plant, we may find out much that will 
be useful to us in the study of Physiological Botany. 
If, for instance, we place some Protococci in rain- 
water, and watch them from day to day, we shall 
observe that they thrive well, and multiply rapidly. 
This proves that the Protococcus can build up its 
cell-wall and protoplasm out of the substances con- 
tained in the rain-water. Now the only substances 
found in fresh rain-water are carbonic anhydride, 
ammonium nitrate, and a-few mineral salts which 
were suspended in the air as dust; and a chemist 
would tell us that the protoplasm consists of a sub+ 
stance called protein, fat, and mineral salts, whilst 
the cell-wall is composed of cellulose and a few of 
the mineral salts. This chemist may be a very 
clever man, but if you ask him to make some pro- 
tein, fat, or cellulose out of the carbonic anhydride, 
ammonium nitrate, &c., he will honestly acknow- 
ledge his inability to do so, because protein, fat, and 
cellulose are organic compounds, and only to be 
found in animal or vegetable substances. From this 
we are forced to the conclusion that some remarkable 
