about 1/8 in. of soil.  Over this, a pane 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 (see Fig. 5) of the plant is different from that of the Cereus, and is more like that of a cucumber.  Still, though the form is different, the purpose of the two cotyledons and the juicy stem in the seedling Opuntia is the same as in the Cereus; and, as the growing point develops, the cotyledons shrivel up and fall off, the plant food they contained having passed into that part of