28 
POPULAR SCIENCE REVIEW. 
wrinkled gelatinous or very light dry lumps or balls, may 
frequently be found on damp stones, earth, or moss, gravel- 
paths, &c. Placed under the microscope, it is seen that these 
are not homogeneous masses, but that they contain within 
them a number of necklace-like threads of cells, which form 
the true Nostoc , a number of individuals being collected into a 
colony surrounded by the gelatinous envelope. In some of the 
genera the cells of which the necklace consists are of two 
different forms, the greater number are green and capable of 
division ; between these occur at intervals, or at the ends of the 
threads, colourless cells of considerable size and incapable of 
division, called Heterocysts. These latter are probably con- 
nected with a yet undiscovered process of reproduction. The 
ordinary mode of propagation is thus described by Thwaites : — 
The jelly which surrounds the colony becomes softened by 
water, the portions of the threads that lie between the hetero- 
cysts become detached, separate from the jelly, and straighten 
themselves, the heterocysts themselves remaining in the jelly. 
The portions of the threads which become detached are en- 
dowed, after entering the water, with spontaneous motion ; the 
cells now grow transversely to the axis of the filament, become 
disc-shaped, and finally divide into threads at right angles to 
the original one, and thus form new Nostoc-filaments, which 
then become enclosed in a new gelatinous envelope. Some 
species of Nostoc are parasitic, these motile threads having 
been seen by Janczewski * to enter the stomata of certain 
species of Hepaticse, of which they were long thought to be 
peculiar vegetative reproductive organs, and being also known 
to inhabit the large cells in the leaves of Sphagnum. The 
species of this family are very widely distributed. Several are 
common in this countty. Nostoc edule is dried by the Chinese, 
and forms an ingredient in their soup. One species abounds to 
such a degree in the Polar regions as to afford an article of food, 
consisting of a modification of cellulose without any deleterious 
admixture. It is produced on soft and boggy slopes bordering 
the sea, and is carried by the wind in all directions, rolling 
along the surface.- Another species was found by Dr. Hooker in 
Kerguelen’s Land, near the southern pole, while several occur 
on the surface of the salt lakes of Thibet, and others in the hot 
springs of India. 
Volvocinece is a small family including only a few genera 
( Volvox, Stephanosphcera , Gonium , and a few others) which, as 
the name implies, are in a constant rolling motion. Each 
individual plant consists of either a single cell or a number of 
* 11 Ann. des Sci. Nat.,” 1872, p. 306, and 11 Quar. Jour. Mior, 8ci.,” .1872, 
p. 367. 
