114 A NURSERY FOR THE MICROSCOPE. 



an advantage to them in the peculiar conditions 

 by which they are surrounded. Not unfrequently 

 their small size is more than compensated for by the 

 enormous rapidity with which individuals are pro- 

 duced. Many of the lowest types of vegetable life 

 with which every tarn, pond, and stream is crowded, 

 and which may be kept with the utmost ease ready 

 for inspection in the aquarium, are single-celled. But 

 these single cells are constantly splitting into two 

 parts, as in the Desmids and the Diatoms, each of 

 which becomes a new individual, and goes through 

 the same mysterious self-division. The main differ- 

 ence between these peculiar objects and vegetable 

 species of a higher organisation and greater magnitude 

 seems to us to consist in the fact that in the former 

 the cells are detached as fast as they are formed, 

 whereas in the latter they adhere together, and thus 

 produce objects of large volume. This is proved by 

 the fact that all species of desmids and diatoms are 

 not single cells. Not unfrequently we find them living 

 in colonies, either for the whole or part of their lives. 



Few objects are prettier than the microscopic plants 

 we are now referring to. Seen by the naked eye their 

 presence is perhaps only revealed by the green film 

 covering the inside of the glass, to which aquarium 

 keepers who are not microscopists strongly object. 

 Desmids and diatoms often cover the stems and 

 leaves of aquatic plants with a greenish or olive- 

 coloured slime, such as Hyalotheca dissiliens (Fig. 69). 



