SPIROGYRA 315 
abundant fresh water plants, and in the examination of other 
forms of fresh water Algae with the microscope one usually finds 
some Desmids present. The cell is peculiar in being organized 
into symmetrical halves, which are separated by a constriction 
that forms an isthmus. The nucleus is in the isthmus, and in 
each half there is a chloroplast and a number of pyrenoids. 
They reproduce in two ways, by cell division and by zygo- 
spores. In multiplying by cell 
division, the cell divides at the 
isthmus, the halves separate, and 
the portion of the isthmus re- 
maining to each half develops a 
new half and thus a new individ- 
ual is formed. In sexual repro- 
duction the cells pair and the 
protoplasts, which escape through 
ruptures at the isthmus, fuse and 
form a zygospore. Sometimes 
the cells after pairing become con- 
nected by a tube through which 
the protoplasts reach each other. 
In either case the entire proto- 
plasts of cells conjugate. 
Spirogyra. — Spirogyra (Fig. 
271), very common in ponds, 
sluggish streams, and watering 
Fic. 271. — A species of Spiro- 
gyra. A, a portion of a filament 
showing a vegetative cell with its 
troughs, is the most familiar fila- 
mentous form of the Conjugales 
and the one most commonly 
studied in elementary classes. It 
gets its name from its large and 
beautiful spiral chloroplasts. Its 
spiral chloroplasts (c) and nucleus 
(n) (X 100); B, filaments conju- 
gating and two zygospores (z) 
fully formed; C, a zygospore 
germinating and producing a new 
filament (X 150). A and B from 
nature, and C from Wolle. 
cells are all alike and it pro- 
duces no zoéspores. Its sexual reproduction, in which the gam- 
etes reach each other through tubes, is its important feature. 
Under certain conditions, filaments pair and line up side by side. 
In this position, the cells of the filaments grow toward each 
other in tubular projections which unite and form open passage 
ways between the cells of the paired filaments. The protoplasts 
of one filament pass through these tubes and fuse with the pro- 
