478 LIFE-HISTORIES 
the process of fission, by which they multiply under ordinary con- 
ditions. As shown in Fig. 308, the outer firm layer of the wall, 
ruptures at the place of constriction, allowing the semicells to 
separate, while the protoplasm of the isthmus or neck which joims 
them elongates, being still covered by a thin, elastic, ner layer of 
cellulose. The nucleus and chromatophores divide, half going to 
either end. A partition is then formed across the middle of the 
isthmus, and the new part on either side grows larger and rounder 
until finally two separate and complete cells are formed each with 
a new and an old half. Meanwhile chromatophores pass over into 
this new semicell, and its outer wall becomes thickened and-sculp- 
tured like the other. In some desmids after fission the cells remain 
attached forming a row comparable to the chain in Nostoc. A 
striking peculiarity of the free forms like Cosmarium is their power 
of locomotion. While not rapid, the movement is quite perceptible 
under the microscope. It has been found to be accomplished by 
protrusions of mucilaginous material, and its direction to be in- 
fluenced by light. Resting spores are produced in the peculiar 
manner shown in Fig. 309. Two cells lying side by side, each 
rupture the outer wall across the middle and the halves separate 
as for fission, but instead of forming a cross partition the protoplasm 
of each plant flows out of the old cell-wall and toward the other 
protoplast. When the two protoplasts come in contact they fuse 
into a single spherical mass, the two nuclei merge into one, the 
chromatophores disppear, and the whole contents becomes brown- 
ish, while the outer cell-wall or exospore ! thickens and puts forth a 
number of projections. In this condition the spore is comparatively 
resistant of cold or dryness. On the return of favorable conditions 
germinations take place as shown in Fig. 310. The contents swell, 
rupture the exospore, and emerge, surrounded by the delicate inner 
wall or endospore. Soon a division of the nucleus takes place fol- 
lowed by division of the protoplast into halves, which become 
constricted, turn green and form cell-walls much like the uniting 
pair from which they were derived. Freed from the endospore the 
newly formed desmids swim about and multiply by fission, during 
which process the chromatophores scon become distinct and the 
cell-wall takes on its characteristic sculpturing. 
A spore formed as above described by the union of two similar 
protoplasts is termed a zygospore,* the uniting protoplasts are known 
as gametes,* and the process of their union is called conjugation.® 
Closely related to the desmids are the ‘‘pond-scums,” algze which 
form tangled masses of delicate threads floating near the surface of 
quiet fresh water. A very common genus is Spirogyra (Fig. 311) 
1 Ex’o-spore < Gr. exo, outside; sporos, spore. 
2 En’do-spore < Gr. endon, within. 
3 Zy’go-spore < Gr. zygon, yoke. 
4Gam/’ete < Gr. gametes, a spouse. 
5 Con’’ju-ga’tion < L. con, together; jugare, join or yoke. 
