NOSTOC.—SPIROGYRA. 398 
but usually there are also placed at definite distances from one 
another larger colourless cells, which are not, like the others, 
capable of division, and are ordinarily known as heterocysts 
(jig. 861). By means of the growth and subsequent division of 
the smaller cells, the Nostoc colony may become increased in 
size, and new colonies also at certain times become formed in 
the following manner. By means of the imbibition of water 
the jelly of the old colony swells up and allows the Nostoc fila- 
ments or rows of cells to become free. Each cell subsequently 
grows rapidly in a transverse direction till the appearance is 
presented by each filament of a number of disc-like bodies 
placed side by side. Cell-division next takes place in a direc- 
tion parallel to the axis of the filament, so that a number 
Fic. 862. Fic. 863. 
Fig. 862. Two filaments of Spirogyra about 
to conjugate; each cell is seen to con- 
tain chlorophyll arranged in spiral 
bands with grains of stareh, oil glo- 
bules, and a central nucleus, surrounded 
by protoplasmic threads which extend 
to the cell-wall. «a, b, e. Lateral pro- 
trusions of the cell-walls of adjoining 
cells, After Sachs. Fig. 863. A. Fila- 
ments of Spivogy7'a conjugating. a. For- 
mation of zygospore. b, Formed zy- 
gospore. B. A filament in which are 
young zygospores, ¢, and which contain 
drops of oil. After Sachs, 
of septate thread-like bodies are produced, which, joining by 
their ends, grow so as to ultimately produce a new Nostoc colony. 
At the same time the heterocysts are developed from cells which 
previously differed in no apparent respect from the rest, and the 
jelly-like envelope of the colony becomes also gradually formed. 
Spirogyra, our next example of this order of plants, is one 
in which the process of reproduction known as conjugation 
very commonly takes place. Spirogyra is an Alga which may 
be found in great quantities in most ponds towards the end 
of summer, and is one of the prettiest objects which can be 
