of glass may be placed, and should remain till the
seedlings appear above the soil. Should the position where the seeds are
to be raised be in a room window, this pane of glass will be found very
useful in preventing the dry air of the room from absorbing all the
moisture from the soil about the seeds. For the germination of Cactus,
and indeed of all seeds, a certain amount of moisture must be constantly
present in the soil; and after a seed has commenced to grow, to allow it
to get dry is to run the risk of killing it.

[Illustration: FIG. 4.--SEEDLINGS OF CEREUS. a, One month after
germination. b, Two months after germination. C, Three months after
germination. (Magnified six times,).]

The seeds of Cactuses may be sown at anytime in the year; but it is best
to sow in spring, as, after germinating, the young plants have the
summer before them in which to attain sufficient strength to enable them
to pass through the winter without suffering; whereas plants raised from
autumn-sown seeds have often a poor chance of surviving through the
winter, unless treated with great care. The seeds of all Cactuses are
small, and therefore the seedlings are at first tiny globular masses of
watery flesh, very different from what we find in the seedlings of
ordinary garden plants. The form of the seedling of a species of Cereus
is shown at Fig. 4, and its transition from a small globule-like mass of
flesh to the spine-clothed stem, which characterises this genus, is also
represented. At a we see the young plant after it has emerged from the
seed, the outer shell of which was attached to one of the sides of the
aperture at the top till about a week before the drawing was made. At b,
the further swelling and opening out, as it were, of what, in botanical
language, is known as the cotyledon stage of development, will be seen;
a month afterwards, this will have assumed the shape of a very small
Cereus. It is interesting to note how the soft fleshy mass which first
grows out of the seed is nothing more than a little bag of food with a
tiny growing point fixed in its top, and that, as the growing point
increases, the food bag decreases, till finally the whole of the latter
becomes absorbed into the young stem, which is now capable of obtaining
nourishment by means of its newly-formed roots.

[Illustration: FIG. 5.--SEEDLINGS OF OPUNTIA, SHOWING MODE OF
GERMINATION. (Magnified three times).]

In the genus Opuntia, the cotyledon stage (se