4 MICROSCOPIC PLANTS. [CHAP. 



cell rapidly along 1 . 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 



