480 LIFE-HISTORIES 
in which the plants are unbranched filaments consisting of a single 
row of comparatively large cylindrical cells containing spiral, 
ribbon-like chromatophores, rather prominent pyrenoids, and co- 
pious cell-sap. Elongation of the thread results from fission of 
the cells much as in Nostoc; but with Spirogyra the more intimate 
union of adjacent cells gives rise to a multicellular individual rather 
than to a colony of unicellular plants. Conjugation takes place 
between cells of separate plants growing near together. Outgrowths 
from two opposite cells meet, and by absorption of the walls at the 
point of contact form a tube connecting the cavities. Through this 
“SE 
O74 
CF if. 
Fic. 312.—Pond-scum. Germination of zygospore. I, resting zygospore; 
f, yellowish-brown layer of the cell-wall; e, outer layer of wall. II, the 
protoplast emerging from the old wall, covered by a thin wall (g) of 
its own. III, young plant beginning to form a thread-like row of cells 
by elongating and forming cross-partitions (w, w’); its small end (d) 
still within the old spore-wall, enclosed by the old cell-wall of the 
original plant whose conjugating-tube (c) is still visible. Much magni- 
fied. (Pringsheim.) 
tube the gamete from one cell passes into the other cell to form a 
zygospore. Germination takes place in the manner indicated by 
Fig. 312. 
What is here especially noteworthy is that in the conjugation of 
Spirogyra we have a simplest form of sexual reproduction. Whereas 
in Cosmarium the gametes are just alike, in Spirogyra the one which 
passes over might be called male, and the other, female, although 
before conjugation no difference between them is perceptible. 
Somewhat more highly developed in both the vegetative 
and the reproductive system are the green algz of the genus 
Ulothrix (Fig. 313) which consist of cylindrical cells forming 
an unbranched filament fastened by one end to a rock or other 
firm support. These filaments grow so crowded. as to form 
