34 PLANT-LIFE 



shape. In this mode of multiplication one-half of 

 the cell - wall of the new individual must needs be 

 older than the other. In the sexual mode of increase, 

 two cells are attracted to each other; they surround 

 themselves with a protective gelatinous sheath. They 

 either come into immediate contact or join by means 

 of a short tube formed for the purpose. The cell- walls 

 split at the constriction, allowing the cell-contents to 

 escape. These contents may blend either in one of the 

 cells or in the tube uniting the cells. The result is the 

 formation of a zygote (Gr. zygon, a yoke). The zygote 

 surrounds itself with a membrane, which is frequently 

 studded with spines. It is not until the germination 

 of the zygote is about to start that the nuclei of the 

 conjugating cells fuse. In the course of germination, 

 the membrane of the zygote bursts, and the contents 

 divide into from two to eight cells, which gradually 

 develop into adult Desmids. 



The Desmids, then, display sex. Probably the 

 asexual method of multiplication preponderates, but 

 sex is in evidence ; it may increase the vigour of the 

 stock as well as numbers. But, as in the Diatoms, there is 

 no differentiation of sex among the Desmids. However, 

 there is somewhat of a distinction between the auxo- 

 spore of the Diatom and the Desmid zygote ; the former 

 is a means of return to normal size, while the latter 

 leads to an increase in numbers. 



The Desmidiaceae, on account of their peculiar sexual 

 mode of reproduction, are classed, with other green 

 fresh-water alga?, as Conjugatae. The Zygnemace^e be- 

 long to the same class, and are no less interesting. This 

 order includes the genus Spirogyra, of which there are 



