394 REPRODUCTIVE ORGANS OF ALGA.—VAUCHEPIA. 
examined under the microscope. Seen with the naked eye, it con- 
sists of a mass of long, very slender green threads or filaments, 
which float in the water where they are growing. Examined 
with the microscope, each filament is seen to be more or less 
cylindrical, and composed of a great number of similar cells 
placed end to end (jig. 862). The chlorophylls arranged in 
the parietal layer of protoplasm of the cell in a definite spiral 
manner ; the name of some of the species being determined by 
the number of such spirals in asingle cell. Each cell is capable 
of growth and division, and by this means the bulk of the entire 
plant is increased. 
When con‘ugation is about to take place, two filaments ap- 
proach each other, and from the sides of contiguous cells ( jig. 
862, a, b, c), protrusions of the wall occur which meet in the 
centre. The protruding ends of the wall then intervening 
between the cavities of the two conjugating cells next become 
absorbed (jig. 863, A, a), and the protoplasm of one cell separates 
itself from its cell-wall, and gradually travels into the other cell, 
where it becomes intimately mixed with the protoplasm existing 
there ; the whole mass then becomes of a somewhat oval shape, 
surrounds itself with a cell-wall, b, and in fact constitutes what 
is called a zygospore. Later on its colour changes from green 
to that of a deep red, and after remaining dormant during the 
winter the zygospore germinates at the beginning of spring, 
and so gives rise to a new Spirogyra plant. 
Vaucheria, which we will now consider, exhibits true sexual 
reproduction, in addition to the formation of asexual spores. 
An irregular kind of alternation of generations exists in this 
genus, inasmuch as asexual spores are usually produced by a 
certain number of successive generations, the sexual process only 
taking place in generations separated by a considerable interval 
from one another. At the same time it must be noticed that 
asexual spores may be formed in the same plant as that in which 
sexual reproduction takes place. 
Vaucheria may be found growing either in water or on moist 
surfaces. Its thallus consists of one very elongated and greatly 
branched cell, attached to some fixed object by means of a por- 
tion of its thallus, which is much branched and perfectly trans- 
parent (fig. 864, p, wv). The other, or non-transparent portion 
of the cell contains protoplasm, chlorophyll grains, and fre- 
quently numbers of small oil globules. The asexual spores are 
formed in various ways in the different species, the more com- 
mon method being that in which a small branch becomes sepa- 
rated from the parent cell by division, the protoplasm thus shut 
off secreting a cell-wall round itself, and thus forming a spore, 
which ultimately germinating gives rise to a new Vawcheria 
thallus. (M. C. Cooke and Bates have described the main 
filaments or threads as much divided off by septa into cells 
at the period of fructification ; and Cooke believes, from this 
