270 



ALG.'E 



broadening of the isthmus 



formation, during the motion, of a filament of mucilage by which the 

 ■desmid is temporarily attached to the bottom, and which gradually 

 lengthens. The movements of desmids are especially vigorous when 

 they are in the act of dividing. Stahl found that, like the movements 

 of zoospores, they are affected by light. 



Vegetative propagation takes place by division or fission, a process 

 which can be easily followed out in species of Cosmarium or Staurastrum, 

 the whole being completed in the course of a few hours. When cell- 

 division is about to commence, the endochrome retreats slightly from 

 the band or ' isthmus ' which connects the two half-cells with one an- 

 other; and the two halves then separate from one another, retaining 

 their connection only by a transverse band formed by the gradual 



this is after a time divided into two by 

 a septum along the length of 

 the isthmus, midway between the 

 two half-cells and parallel to the 

 constriction between them. The 

 endochrome now passes out of 

 each original half-cell into the 

 half of the band in connection 

 with it, and at the same time the 

 half-band bulges, and, growing 

 rapidly, assumes the form and 

 appearance of an original half- 

 cell. Fresh formation of chloro- 

 phyll is at the same time taking 

 place in it, and the half-band becomes a complete half-cell, but some- 

 times slightly larger. We have now two individuals attached to one an- 

 other by their larger halves ; these frequently remain in contact for a 

 considerable period, but at length separate. In the spiny species of 

 Staurastrum the spines are developed very rapidly on the half-bands 

 while their development into half-cells, is progressing. 



A sexual process of conjugation takes place in the following way 

 in the genera where the individuals are quite distinct. Two individuals — 

 which cannot in any way be differentiated as male and female — lay them- 

 selves either parallel to or across one another, and the pair become en- 

 veloped in a common mucilaginous coating. In each individual the 

 outer of the two layers of which the cell-wall is composed gives way, and 

 a circular opening is formed at the constricted part ; the inner layer of 

 the cell-wall of each individual protrudes through the opening in the 

 form of a bladder, and these two protrusions come into contact. The 

 outer cell-wall is then thrown off, and the wall separating the two con- 



Fic. 240. — Staiirastriivi telifcrjun Ralfs, dividing 

 (x 400). (From nature.) 



