THALLOPHYTA 339 
row of conspicuous pyrenoids. A single nucleus is in the center 
of the cell and is frequently conspicuous. The nucleus is sur- 
rounded by cytoplasm, and cytoplasm also lines the cell wall and 
extends as fine strands from the cytoplasm around the nucleus 
to the peripheral cytoplasm. The cells of Spirogyra are alike 
not only in structure but also in function. Each cell performs 
all the vegetative functions of the plant. Every cell absorbs 

Fie. 346. Spirogyra 
Successive stages in conjugation and formation of zygospore (x 165) and 
germination of zygospore 
water, carbon dioxide, and mineral matter; every cell carries on 
photosynthesis; and every cell has the power of growing and 
dividing. A filament grows by the division of its cells and the 
subsequent elongation of the daughter cells. One filament may 
break into two and thus form two filaments. 
Reproduction. Spirogyra reproduces sexually by conjugation, 
which is the union of two similar or nearly similar cells to form 
a zygospore. In most species this process is initiated by two 
filaments coming to lie side by side. Projections then grow sin- 
gly from the cells of each filament toward those of the opposite 
