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 Euglena 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 flagellum 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, 11, 12, and 13. Fig. 7 
is that of a desmid (Cosmarium) in the act of divi- 
* See ** Science Gossip,” 1879, pp. 159, 231. 
