42 



CONJUGATE. 



individuals ; sometimes akinetes or aplanospores are formed in 

 addition. They only occur in fresh or slightly brackish water. 



Order 1. Desmidiaceae. The cells generally present markings 

 on the outer wall, and are mostly divided into two symmetrical 

 halves by a constriction in the middle, or there is at least a 

 symmetrical division of the protoplasmic cell-contents. The cell- 

 wall consists nearly always of two layers, the one overlapping 

 the other (Fig. 35 0). The cells either live solitarily or are united 

 into unbranched filaments. The mass of protoplasm formed by 

 the fusion of the two conjugating cells becomes the zygote, which 

 on germination produces one (or after division 2, 4 or 8) new 

 vegetative individual. The chromatophores are either star-, plate-, 

 or band-shaped, and regularly arranged round the long axis of the 

 cell. 



A 



FIG. 35. A Cell of Gi/wiiozy.oa brebistonii, external view showing the distribution of the 

 pores. B A portion of the membrane of Slaurastrum licorne with pores containing proto- 

 plasmic projection?. Cell-wall of Hyalotheca inucosa during cell-division ; the central 

 part, being already formed, shows the connection with the divisional wall. 



The Desmidiaceee are not able to swim independently, many 

 species, however, show movements of different kinds by rising and 

 sliding forward on the substratum. These movements, which are 

 partly dependent upon, and partly independent of light and the 

 force of gravitation, are connected with the protrusion of a mu- 

 cilaginous stalk. The mucilage, which sometimes surrounds the 

 whole individual, may acquire a prismatic structure, it is secreted 

 by the protoplasmic threads which project through certain pores 

 definitely situated in the walls (Fig. 35 A, B). 



VEGETATIVE MULTIPLICATION takes places by division. A good 

 example of this is found in Cosmarium botrytis (Fig. 36 A-D). 

 The nucleus and chromatophores divide, and simultaneously the 

 central indentation becomes deeper, the outer wall is then ruptured 

 making a circular aperture through which the inner wall protrudes 

 forming a short, cylindrical canal between the two halves to which 

 it is attached (Fig. 36 C). After elongation the canal is divided 

 by a central transverse wall, which commences as a ring round its 



