TYPES OF CRYPTOGAMS; THALLOPHYTES 



243 



the division of the protoplasm and formation of a partition of 

 cellulose in a kind of pond-scum 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. 178). Two cells (apparently 

 precisely alike) come in contact, 

 undergo a thinning-down or absorp- 

 tive process in the cell-walls at the 

 point of contact, and finally blend z . 

 their protoplasmic cell-contents, as 

 shown in the figure, to form a mass 

 known as a spore, or more accu- 

 rately a zygospore, from which, after 



i IT 



FIG. 178. Conjugation of Cells of Green Algaj. (Much magnified.) 



I. Conjugation of Desmids. A, a single plant in its ordinary condition; , empty 

 cell-wall of another individual ; C, conjugation of two individuals to form a 

 spore by union of their cell-contents. 



II. Conjugation of Spirogyra. A, two filaments of Spirogyra side by side, with 

 the contents of adjacent cells uniting to form spores, z. At the bottom of the 

 figure the process is shown as beginning at the top as completed, and the cells 

 of one filament emptied ; B, a single filament of another kind of Spirogyra, 

 containing two spores, one lettered z. (A magnified 240 diameters, B 150 

 diameters.) 



a period of rest, a new individual 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, 

 since the spore-formation takes place at night. It is possible, 



