The Corolla.—If from the circumference we proceed to the centre of the flower, the calyx being the first, the corolla will be its second envelope.
If, on the contrary, we proceed from the centre to the circumference, the corolla will form the fourth whorl; the pistil (consisting of stigmata, stylus, and ovary) being the first; the nectariferous disc (often wanting) the second; and the stamens the third whorl. Remarkable for its varied tints, the corolla, to indifferent or ignorant eyes, seems the entire flower.
A black or blackish colour is exceedingly rare. Out of 300 vegetable species which compose the flora of Central Europe, there are not six with blackish or even grayish flowers. No hypothesis has yet been put forward to explain this mark-worthy rarity.
Species with a yellow corolla are the most numerous, forming more than a sixth of the European flora: then come, in their order of frequency, species with green, white, red, and blue flowers; the white increasing in number as we approach the Pole.
Dividing the flora into twenty parts, we may ascribe to each colour, and its various tints, the following proportion:—
| Yellow, | 6.0 |
| Green, | 4.50 |
| White, | 4.0 |
| Red, | 3.50 |
| Blue, | 1.50 |
| Black, | 0.50 |
The analogy between the parts or petals of the corolla and the leaf, is perhaps not quite so striking as between the leaf and the sepals of the calyx. The phrase "rose-leaves" is an expression consecrated by immemorial usage. Why not prefer the term "corollary leaves or leaflets (foliola)" to that of petals?
The corollary leaves, or petals, are organised like true leaves. They have the same system of venation; their lamina correspond to the "limb," or "blade;" and their unguis, or "claw," to the "petiole," or stalk. (See Fig. 46, a.) The upper margin of petals is frequently more obtuse than the overspreading margin of the blade of a leaf, which, in most cases, is pointed. Non-unguiculate petals represent the sessile leaves. (Fig. 46, b.) Their form is much more varied than that of the calicinal foliola, which are never unguiculate or petiolated.
