DESMIDIACE.E. 73 



formed, and which is known as conjugation. In this 

 process the opposite cells of two distinctfilaments, lying 

 near together, push out protrusions of the cell-walls, 

 which meet and open into each other, forming cross 

 tubes, as in Spirog'yra nit'ida (PL V. fig. 12). The 

 contents of the opposite cells of the filaments then 

 unite, forming large spores, which remain either in 

 the cells of one of the filaments or in the cross 

 tubes. 



The three species figured are common in clear 

 pools. 



DESMIDLA/CE^E. The Desmidiacese are truly micro- 

 scopic, few of them being even perceptible to the naked 

 eye without the very closest examination. They are 

 very beautiful, on account of their bright green co- 

 lour and often elegant forms. Many of them are 

 very common, existing in every pond or ditch; but 

 they abound most in clear open boggy pools on heaths. 

 On placing some water containing them in a glass 

 jar and exposing it to the light, they will often be 

 found adhering to the glass, or forming a layer on 

 the surface of the muddy sediment. 



The Desmidiacese consist mostly of single cells 

 (PL V. figs. 9, 16) ; and these consist of two equal halves 

 or segments, as indicated either by a paleness of the 

 endochrome or a deep constriction at the line of 

 junction, which is called the suture. The cells are 

 often elegantly lobed and cut, or spiny; and in 

 many the surface exhibits minute markings, consist- 

 ing of little protrusions of the cell-wall outwards, or 

 inflations, as they are called. 



Their reproduction is effected by division and con- 

 jugation. In the process of division the cells gradually 

 separate at the suture, and a new half-cell is formed 

 upon each old half, which grows until it attains the 

 size and form of the original half of the parent-cell. 

 The conjugation is effected by two cells approximating 

 so that their sutures are near together, the cells then 



