leaf-blades are seen, one on each side of it. All the rest are the bulb-scales; they completely wrap round each other, and are called tunicated, so, when cut across they look like rings. They are really the bottom parts of the leaves without the blades above. a, a are little bulbs or bulbils, which, when sufficiently large, will fall off and become new plants. The bulk of the bulb is therefore composed of bulb-scales. There is a kind of onion growing in the intensely hot sand of the desert near Cairo, which, to keep the innermost scales fresh, hardens the outer ones till they feel like wooden coverings. The temperature of the sand may be 130° Fahr.

Many bulbs have scales which do not surround the stem, but grow like the scales of a fir-cone, overlapping one another, as do those of lilies.

A corm, as shown in Fig. 22, of a Gladiolus consists mainly of the globular base of the stem, b; the scales, being dried up, form a thin covering, d, с being the flowering stem in the middle. The new corin, b, is formed on the top of the old one, a. This supplied the nourishment for the flower, and then shrivelled. The fresh leaves then set to work to make starch, which is stored up in the cells of the new corm, which keeps getting larger to receive it.

Fig. B is the corm with the skins removed; a, b, c, d, are the same as in Fig. A, but y is a bud on the top of the new corm, which will flower next year;

z is an extra little corm arising from the new one; x are the roots.

Some of the corms are edible, and called "Uintjes." The cultivated leek is an instance of a plant which has a bulb when wild, but has lost it under cultivation. In many plants the roots swell into nodules, and

FIG. 22.-Corm of Gladiolus. A, Longitudinal section: a, last year's, b, this year's corm; c, flower stem; d, scales; x, roots. B, After removal of the enveloping scales: y, bud, which will develop into next year's corm; z, bulbil.

act as reservoirs of water and nourishing matters. Fig. 23 illustrates a South African kind of Asparagus which has swollen nodules on its roots, most probably for this purpose.

Fig. 24 shows an elongated form of root. This closely resembles the swollen end of the underground stem of Oxalis cernua, described above.

It is, then, by making bulbs, etc., that so many plants can manage to "adapt" themselves to endure the dry season.

Similarly, it is by "acquiring," as we say, fleshy stems, as of Euphorbias, or fleshy leaves, as of the

Hottentot fig and many others, that they can live and thrive in South Africa.

The peculiarities of leaves, mentioned in the last chapter, have likewise all been acquired in adaptation to the prevailing drought.

It was Darwin who gave us the clue to the "origin

FIG. 24.-Anthe'ricum, with long fleshy

roots.

of species" as above described. He called this influence of the surroundings "the direct action of the conditions of life," the result, or effect, of which was, that all the plants affected by it varied alike, i.c. in adaptation to the new conditions of life, and so a new variety" would arise.

THE STRUCTURE OF FLOWERS.

THE next matter for consideration is to discover how differences can arise in the structure of flowers. These may be grouped under the following headings: Number, Cohesion, Adhesion, Insertion, and Form.

Number. With regard to the number of parts in each whorl, they may vary from one to twelve-all such numbers are said to be definite (see Figs. 6, 7, 8, 12); but if there are more than twelve, sometimes very many, such are regarded as being indefinite (Fig. 9, stamens many; Fig. 10, petals and stamens many), and is represented by the mathematical symbol for infinity, ∞. Some numbers are commoner than others, as fives and threes; fours are less common, and twos are rare. The multiples of these occur, so that there may be eight, ten, or twelve in a whorl.

Any one, two, or more whorls may be wanting altogether; so that a flower may be reduced to stamens (Figs. 6, I.; 8), or a pistil only (Figs. 5, II.; 6, II.). On the other hand, the calyx and corolla may exist without the stamens or pistil. Such a flower is then said to be neuter, as of the garden Hydrangeas.