226 GENEBAL BOTANY 



wall at the middle of the mother cell (Fig. 118). As the ridge 

 grows by the deposit of new cellulose on its inner edge it 

 projects farther and farther into the cell like a thin disk, or 

 plate, except that the center of the disk is not yet completed. 

 As a result of this growth of the cell wall the protoplasmic sac 

 is pushed in and constricted before the advancing wall. Finally 

 the entire sac is cut off by the completion of the disk, and the 

 new wall forms a complete plate across the old cell, dividing it 

 into two new daughter cells. During this process the nucleus 

 divides into two equal parts, each of which organizes a new 

 nucleus in one of the daughter cells. The daughter cells and 

 their nuclei then grow to the size of the mother cell, and in this 

 manner the filaments elongate. 



Reproduction. Vegetative reproduction occurs when the delicate 

 filaments are broken up by the waves or are separated into single 

 cells or cell fragments. Since each cell of Spirogyra is capable 

 of maintaining life and growth independently of the other cells 

 of the filament it follows that individual cells or fragments of 

 filaments are able to elongate into new plants and thus to 

 multiply the number of individuals indefinitely. 



Sexual reproduction takes place regularly either in the early 

 spring or in the fall, before the plants enter upon the winter 

 period of rest. Bright sunshine and abundant rainfall appear to 

 be factors in inducing Spirogyra to resort to sexual reproduc- 

 tion, while the opposite conditions, with abundant nutriment in 

 the water in which the plants live, tend to prolong the growth 

 period for vegetative reproduction. This is doubtless due to a 

 stimulus exerted by light and soluble food salts upon the deli- 

 cate protoplasts of the filaments, which respond by initiating 

 reproductive or vegetative growth processes, just as the cells of 

 a root may respond by tropistic movements to the influence 

 of gravity. 



G-ametophyte is the term applied to the plant filaments of Spiro- 

 gyra which reproduce and form new offspring by means of 

 gametes. When such plants are about to reproduce sexually, two 

 filaments which lie near each other become united by tubular out- 

 growths formed by protrusions from their cells (Fig. 119, a). 



