8 MICROSCOPIC PLANTS. [CHAP. 



is perfectly green, owing to their presence in vast 

 numbers, but you can seldom place reliance upon 

 any particular pond for containing it. It is to be 

 found during all seasons when the rains of spring 

 have swollen the ponds, when the summer or autumn 

 sun has reduced their circumference, and when they 

 are covered with two or three inches of ice they 

 may still be found. But you may find Volvox in 

 profusion in a pond to-day, whilst in a week, a 

 month, or a year, you may search diligently and long 

 in the same water without the slightest success. 



There is another active little plant you are sure 

 to find in a pond the Engiena viridis. It is of a 

 bright green colour, generally tapering towards each 

 extremity, and having a red spot, like an eye, at one 

 end. This is really not an eye, but due to some 

 change in a portion of its chlorophyll. Sometimes 

 specimens will be found possessing a long delicate 

 filament (flagellum), as in the illustra- 

 tion, and Mr. M. H. Robson* has re- 

 cently called attention to the fact that 

 this flagettum is sometimes bulbed at 

 its extremity (fig. 6, c}. What is the 

 use of this bulb has not been yet 

 explained, but it is suggested it may 

 be used as a sucker to enable it to cling to sur- 

 faces. 



Some of the most beautiful forms will be found 



among the Diatoms and Desmids. A few of these 



will be seen in figs. 7, 8, 9, 10, n, 12, and 13. Fig. 7 



is that of a desmid (Cosmariuni) in the act of divi- 



* See "Science Gossip," 1879, pp. 159, 231. 



