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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 
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 
Fie. 178. — Conjugation of Cells of Green Algze. (Much magnified.) 
{. Conjugation of Desmids. A,asingle plant in its ordinary condition ; B, 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, 
