bodies, called " pyrenoids. " The cell has several nuclei, but they are 

 scarcely evident in the living cell. By placing the cells for a few hours in 

 a one per cent watery solution of chromic acid, then washing thoroughly 



FIG. 13. Cladophora. A, a fragment of a plant, x 50. B, a single cell treated 

 with chromic acid, and stained with alum cochineal, n, nucleus, py. pyre- 

 noid, x 150. C, three stages in the division of a cell, i, 1.45 p.m.; n, 2.55 

 p.m. ; in, 4.15 p.m., x 150. D, a zoospore x 350. 



and staining with borax carmine, the nuclei will be made very evident 

 (Fig. 13, B). Such preparations may be kept permanently in dilute 

 glycerine. 



If a mass of actively growing filaments is examined, some of the cells 

 will probably be found in process of fission. The process is very simple, 

 and may be easily followed (Fig. 13, C). A ridge .of cellulose is formed 

 around the cell wall, projecting inward, and pushing in the protoplasm as 

 it grows. The process is continued until the ring closes in the middle, 

 cutting the protoplasmic body completely in two, and forms a firm 

 membrane across the middle of the cell. The protoplasm at this stage 

 (C in.) is somewhat contracted, but soon becomes closely applied to the 

 new wall. The whole process lasts, at ordinary temperatures (20-25 C.), 

 from three to four hours. 



At certain times, but unfortunately not often to be met with, the con- 

 tents of some of the cells form, by internal division, a large number of 

 small, naked cells (zoospores) (Fig. 13, D), which escape and swim about 

 actively for a time, and afterwards become invested with a cell wall, and 

 grow into a new filament. These cells are called zoospores, from their 

 animal-like movements. They are provided with two cilia, closely re- 

 sembling the motile cells of the Protococcacece and Volvocineoc. 



