MICROSCOPIC OBJECTS. 50 



Fig. 99. A Species of Desmid (Closterium Leibleinii), x 120. 



Desmids also belong to the Confervoid Algse ; they may 

 usually be distinguished from Diatoms, in their natural state, 

 by their green colour. They, as well as Diatoms, occur in 

 large numbers in ponds that have an exposed situation, 

 although they may be found in nearly all standing water. 

 The great variety and beauty of their forms have caused them 

 to be much studied. 



The frustules consist of a cellulose coat or membrane, enclo- 

 sing the cell- contents. Each frustule forms only a single cell, 

 which may be demonstrated by fracturing one end, when all 

 the cell-contents will escape from the frustule, which of course 

 would not be the case if the apparent division in each frustule 

 were real (see fig. 100) . These plants, like the Diatoms, may 

 often be separated from the extraneous matter by the action 

 of light, they always being found to travel towards the lightest 

 part of their habitations. 



They may be mounted in a shallow cell, in camphor-water 

 although it is difficult, if not impossible, to preserve them 

 with their colours &c. uninjured. 



Fig. 100. A Species of Desmid (Micrasterias denticulata) , 



x!70. 



This, like the preceding species of Desmid, is tolerably 

 common. In the centre of the frustule there will be noticed 

 an apparent transverse division, as mentioned at fig. 99 ; but 

 the appearance is delusive, as will be seen on examination. 

 That Desmids have the power of locomotion is apparent to 

 all students of microscopy ; but how they move is at present 

 a mystery. Their reproduction occurs in three or four ways 

 by cell-division (which is the commonest mode), by spores, 

 and by ciliated zoospores. The last mode of reproduction is 

 exceedingly interesting ; it may often be noticed by examina- 

 tion under the miroscope, more especially in the spring and 

 summer months. The Desmid appears to have its cell-con- 

 tents contracted, and within this contraction the really active 

 zoospores will be seen in constant motion. The examination 

 of many species will amply repay the trouble ; for when this 

 mode of reproduction has been seen, the time and trouble 

 have been well spent. 



