SOME TYPES OP PLOWEELBSS PLANTS. 



223 



which result in the division of the protoplasmic contents of a cell into 

 two independent portions, each of which is at length surrounded by a 

 complete cell wall of its own. In Fig. 193 the division of the protoplasm 

 and formation of a partition of cellulose in a kind of pondscum are 

 shown, but the nucleus and its changes are not represented. 



Another kind of reproduction, namely by conjugation, is found in 



Spirogyra. This process in its simplest 

 form is found in such unicellular plants 

 as the desmids, Fig. 194. Two cells 

 (apparently precisely alike) come in con- 

 tact, undergo a tljinning-down or ab- 

 sorptive process in the cell walls at the 

 point of contact, and finally blend their 

 protoplasmic cell-contents, as shown in 

 the figure, to form a mass known as a 

 spore, or more accurately a zygospore, 

 from which a new individual soon 



Fig. 193. — Process of Cell-Multi- 

 plication in a Species of Pond- 

 scum. (Considerably magnified.) 



A, portion of a filament partly 

 separated at a and completely so 

 at 6 ; -B, separation nearly com- 

 pleted, a new partition of cellu- 

 lose formed at a ; C, another 

 portion, more magnified, show- 

 ing mucous covering d, general 

 cell wall u, and a delicate mem- 

 brane a, which covers the cell- 

 contents b. 



Fio. 194, — Unicellular Fresh-Water 

 Plants (desmids), forming Spores 

 by Conjugation, (Much magnified,) 



A, a single plant in its ordinary 

 condition ; -B, empty cell wall of 

 another individual ; C, conjuga^ 

 tion of two individuals to form a 

 ppore by union of their cell-con- 

 tents. 



develops. In Spirogyra each cell of the filament appears to be an 

 individual and can conjugate like the one-celled desmids. It is not easy 

 to watch the process, isinoe the growth of the filaments goes on mainly 

 by day, in sunlight, and the spore-formation takes place at night, when 

 growth is less rapid. It is possible, however, to retard the occurrence of 



