MICROSCOPE AT THE POND-SIDE. 59 



tinguished from the diatoms by their bright-green 

 colour, and by their cells not depositing silex, or 

 flinty matter, as is the case with the latter. The 

 siliceous nature of the shells of diatoms is made 

 apparent by their not being acted on by strong 

 acids, as nitric and hydrochloric. 



The desmids sometimes abound in ditches and 

 small pieces of standing water. Amongst other 

 objects in a drop of water they are easily recog- 

 nized by their beautiful bilateral forms and dark- 

 green colour. One o-f the most charming of these 

 is named Euastrum, and consists of two notched 

 halves of a bright-green colour, with darker green 

 spots. It is represented at figure 28, plate 2. 

 The green matter is composed of a waxy substance, 

 called chlorophyle, and is the same matter as that 

 which produces the green colour of leaves. Some 

 of the desmids assume a lunate form, and are 

 named Closterium, a species of which is figured 

 at 29, plate 2. There are various species of Clo- 

 sterium, all of the same general form, and occa- 

 sionally occurring in very great abundance. Some- 

 times several of the cells are attached together, 

 forming a long chain, as in the genus Desmidium, 

 seen at figure 30, from which the family takes its 

 name. These break up and go on growing. When 

 they grow, the new cells are formed between the 

 two halves of the parent cells. This is represented 

 at figures 136 and 137, plate 5. In a genus 

 called Scenedesmus, several cells are united, and 

 the two last halves are furnished with horns, as 

 seen at figure 32 ; at other times several cells 

 unite, forming a globular mass, as in Pediastrum, 

 represented at figure 31. In this case each cell 

 presents two projections, forming objects of singular 

 beauty. 



The diatoms are more numerous and widely 



