Prof. Huxley avoids this difficulty by making every kind of Genesis a mode of development. His classification, which suggested the one given above, is as follows:—
| Development |  | Continuous |  | Growth Metamorphosis |
Discontinuous | | Agamogenesis Gamogenesis | | Metagenesis Parthenogenesis |
The implication is that an essentially similar process occurs in those fragments of leaves used for artificial propagation. Besides the Begonias in general, I learn that various other plants are thus multiplied—Citron and orange trees, Hoya carnosa, Aucuba japonica, Clianthus puniceus, etc., etc. Bryophyllum calicinum, Rochea falcata, and Echeveria. I also learn that the following plants, among others, produce buds from their foliage leaves:—Cardamine pratensis, Nasturtium officinale, Roripa palustris, Brassica oleracea, Arabis pumila, Chelidonium majus, Nymphæa guianensis, Episcia bicolor, Chirita sivensis, Pinguicula Backeri, Allium, Gagea, Tolmia, Fritillaria, Ornithogalum, etc. In Cardamine and several others, a complete miniature plant is at once produced; in other cases bulbils or similar detachable buds.
Among various examples I have observed, the most remarkable were among Foxgloves, growing in great numbers and of large size, in a wood between Whatstandwell Bridge and Crich, in Derbyshire. In one case the lowest flower on the stem contained, in place of a pistil, a shoot or spike of flower-buds, similar in structure to the embryo-buds of the main spike. I counted seventeen buds on it; of which the first had three stamens, but was otherwise normal; the second had three; the third, four; the fourth, four; &c. Another plant, having more varied monstrosities, evinced excess of nutrition with equal clearness. The following are the notes I took of its structure:—1st, or lowest flower on the stem, very large; calyx containing eight divisions, one partly transformed into a corolla, and another transformed into a small bud with bract (this bud consisted of a five-cleft calyx, four sessile anthers, a pistil, and a rudimentary corolla); the corolla of the main flower, which was complete, contained six stamens, three of them bearing anthers, two others being flattened and coloured, and one rudimentary; there was no pistil but, in place of it, a large bud, consisting of a three-cleft calyx of which two divisions were tinted at the ends, an imperfect corolla marked internally with the usual purple spots and hairs, three anthers sessile on this mal-formed corolla, a pistil, a seed vessel with ovules, and, growing to it, another bud of which the structure was indistinct. 2nd flower, large; calyx of seven divisions, one being transformed into a bud with bract, but much smaller than the other; corolla large but cleft along the top; six stamens with anthers, pistil, and seed-vessel. 3rd flower, large; six-cleft calyx, cleft corolla, with six stamens, pistil, and seed-vessel, with a second pistil half unfolded at its apex. 4th flower, large; divided along the top, six stamens. 5th flower, large; corolla divided into three parts, six stamens. 6th flower, large; corolla cleft, calyx six cleft, the rest of the flower normal. 7th, and all succeeding flowers, normal.
While this chapter is under revision, another noteworthy illustration has been furnished to me by a wall-trained pear tree which was covered in the spring by luxuriant "foreright" shoots. As I learned from the gardener, it was pruned just as the fruit was setting. A large excess of sap was thus thrown into other branches, with the result that in a number of them the young pears were made monstrous by reversion. In some cases, instead of the dried up sepals at the top of the pear, there were produced good sized leaves; and in other cases the seed-bearing core of the pear was transformed into a growth which protruded through the top of the pear in the shape of a new shoot.
In partial verification, Mr. Tansley writes:—"Prof. Klebs of Basel has shown that in Hydrodictyon, gametes can only be produced by the cells of a net when these are above a certain size and age; and then only under conditions unfavourable to growth, such as a feeble light or poverty of nutritive inorganic salts or absence of oxygen, or a low temperature in the water containing the plant. The presence of organic substances, especially sugar, also acts as a stimulus to the formation of gametes, and this is also the case in Vaucheria. Many other Algæ produce gametes mainly at the end of the vegetative season, when food is certainly difficult to obtain in their natural habitat, and we may well suppose that their assimilative power is waning. Where, however, as is the case in Vaucheria, the plant depends for propagation mainly on the production of fertilized eggs, we find the sexual organs often produced in conditions very favourable to vegetative growth, in opposition to those cases such as Hydrodictyon, where the chief means of propagation is by zoospores. So that side by side with, and to some extent obscuring, the principle developed above we have a clear adaptation of the production of reproductive cells to the special circumstances of the case."