232 BOTANY. 



ently pure water placed for a few weeks in a clean bottle and exposed 

 to the light will yield an abundant crop, generally of one species. 



309. Order Zygnemacese. The plants of this order are 

 elongated unbranched filaments, composed of cylindrical 

 cells arranged in single rows. The cells are all alike, and 

 each one appears to be independent, or nearly so, of its asso- 

 ciates. The filament is thus, in one sense, rather a com- 

 posite body than an individual. Each cell has usually a 

 centrally placed nucleus, with radiating extensions of the 

 protoplasm passing from it to the layer lining the inner sur- 

 face of the wall. The chlorophyll is generally arranged in 

 bands or plates, but under certain conditions it exists in 

 shapeless masses. 



310. The vegetative increase of the number of cells takes 

 place by the fission of the previously formed cells. The 

 protoplasm in a cell divides, and a plate of cellulose forms in 

 the plane of division. This is repeated again and again, and 

 by it the filament becomes greatly elongated. It is interest- 

 ing to note that this increase of cells, which here constitutes 

 the growth of the plant-body, is that which in simpler plants 

 is called the asexual mode of reproduction. In the plants 

 under consideration there is barely enough coherence of the 

 cells to enable them to constitute a plant-body, and one can 

 readily see that the same fission of the cells which now takes 

 place, and which here increases the size of the plant, would, 

 if the cells cohered less, simply increase the number of indi- 

 viduals. 



As might be expected, the filaments occasionally separate 

 spontaneously into several parts of a considerable length, 

 and the parts floating away give rise to new filaments. The 

 separation takes place by the cells first rounding off slightly 

 at the ends, so that their union is weakened at their cor- 

 ners ; finally only the centres of the rounded ends are left 

 in slight contact, which soon breaks. 



311. The sexual reproduction is well illustrated in Spi- 

 rogyra, one of the principal genera. At the close of their 

 growth in the spring, the cells push out little processes from 

 their sides, which extend until they come in contact wit!) 



