Jeterus, or jaundice, a general yellowness.

Pernio, or chilblains, wounds caused by frost.

Rachitis, or premature falling of grain.

Verminatio, or being preyed upon by the larvae of insects. Phthiriasis, or lousiness, when the leaves and stems are infested by Aphides, or the like.

Squamatio, or scaliness, when scales are formed instead of leaves.

Exostosis, or clubbing of the roots.

Crispatura, or curling, in consequence of the leaves being punctured by insects.

Decoloration, or loss of colour.

Coloration, or staining.

Anthozusia, or change of the leaves into petals.

CHAPTER XV.

OF HYBRID PLANTS.

It is well known that, in the animal kingdom, if the male and female of two distinct species of the same genus breed together, the result is an offspring intermediate in character between its parents, but uniformly incapable of procreation unless with one of its parents; while the progeny of varieties of the same species, however dissimilar in habit, feature, or general characters, is in all cases as fertile as the parents themselves. A law very similar to this exists in the vegetable kingdom.

Two distinct species of the same genus will often together produce an offspring intermediate in character between themselves, and capable of performing all its vital functions as perfectly as either parent, with the exception of its being unequal to perpetuating itself by seed; or should it not be absolutely sterile, it will become so in the second, third, or, very rarely, fourth generation. It may, however, be rendered fertile by the application of the pollen of either of its parents; in which case its offspring assumes the character of the parent by which the pollen was supplied. This power of hybridising appears to be far more common in plants than in animals; for while only a few animal mules are known, there is scarcely a genus of domesticated plants in which this effect cannot be produced by the assistance of man, in placing the pollen of one species upon the stigma of another. It is, however, in general only between nearly allied species that this intercourse can take place; those which are widely different in structure and constitution not being capable of any artificial union. Thus the different species of strawberry, of certain tribes of Pelargonium, and of Cucurbitaceæ, intermix with the greatest facility, there being a great accordance between them in general structure and constitution; but no one has ever suc

ceeded in compelling the pear to fertilise the apple, nor the gooseberry the currant. And as species that are very dissimilar appear to have some natural impediment which prevents their reciprocal fertilisation, so does this obstacle, of whatever nature it may be, present an insuperable bar to the intercourse of different All the stories that are curgenera. rent as to the intermixture of oranges and pomegranates, of roses and black currants, and the like, may therefore be set invention.

down to pure

By far the best series of observations that has been instituted with a view to determine the laws of hybridism was that of Kölreuter, who, about the year 1775, commenced a set of experiments, which he continued to prosecute for twenty years, upon species of the genera Digitalis, Verbascum, Solanum, Malva, Linum, Dianthus, and Mirabilis. It is upon those experiments, combined with the subsequent experience of others and my own observations, that the foregoing statement has been made.

It has, nevertheless, been asserted by divers experienced cultivators of the present day, that the conclusions drawn from the experiments of Kölreuter have been too hasty; and that if they apply to the genera that were the special subject of the attention of that observer, they are by no means applicable to plants in general. It has been urged, in proof of this statement, that many different species of African Gladioli, of Pelargonium, of South American Amaryllis, of Crinum, of Triticum, &c., breed freely together, and that their seedlings are as fertile as themselves.

I must confess that these instances are by no means such as to shake my confidence in the accuracy of the laws deduced from Kölreuter's experiments. In the first place, there is a degree of vagueness and looseness in all the cases that are specified, which is particularly striking if compared with the precision with which Kölreuter's experiments were conducted; secondly, in all the cases above mentioned, which I believe are the most remarkable, there is much room for doubt whether the supposed species upon which the argument is founded are any thing more than wild varieties of each other. The African Gladioli are known to intermix freely; but

Mr. Herbert, in his account of them, in the Horticultural Transactions, vol. iv. p. 16., admits that he cannot speak to the power of their mules to perpetuate themselves by seed. No botanist can fix positive characters to a large part of the reputed species of Pelargonium, or to the South American Amaryllises, which Mr. Herbert calls Hippeastra; many of the supposed species of Crinum seem to have no better claim to be so considered than the varieties that might be picked from a bed of tulips; and, lastly, the Tritica cærulescens, polonicum, and tomentosum, upon which Bellardi's experiments were founded, are plants with the history of which no man is acquainted, and which, in all probability, derive their origin from the Triticum æstivum, or common wheat.

All I think that can be conceded upon this subject is, that more hybrid plants are fertile to the third or fourth generation than Kölreuter supposed: that they will all, in time, revert to one or other of their parents, or become absolutely barren, there can be no doubt whatever.

The cause of the sterility of mule plants is at present entirely unknown. Sometimes, indeed, a deficiency of pollen may be assigned; but in many cases there is no perceptible difference in the healthiness of structure of the fertilising organs of a male plant and of its parents. I know of no person who has attempted to prove this by comparative anatomical observations, except Professor Henslow, of Cambridge; who, in an excellent paper upon a hybrid Digitalis, investigated anatomically the condition of the stamens and pistillum, both of his hybrid and its two parents, with great care and skill. The result of his enquiry was, that no appreciable difference could be detected.

Although this power of creating mule plants that are fertile for two or three generations incontestably exists, yet in wild nature hybrid varieties are far from common; or, at least, there are few well attested instances of the fact. Among the most remarkable cases are the Cistus Ledon, constantly produced between C. monspessulanus and laurifolius, and Cistus longifolius, between C. monspessulanus and populifolius, in the wood of Fontfroide, near Narbonne, mentioned by Mr. Bentham. Again, the same acute botanist ascertained that Saxi

fraga luteopurpurea of Lapeyronse, and S. ambigua of De Candolle, are only wild accidental hybrids between S. aretioides and calyciflora: they are only found when the two parents grow together; but there they form a suite of intermediate states between the two. Gentians having a similar origin have also been remarked upon the mountains of Europe. It is difficult not to believe that a great number of the reputed species of Salix, Rosa, Rubus, and other intricate genera, have also had a hybrid origin; but I am not aware that there is at present any positive proof of this.

In a practical point of view, I am inclined to believe that the power of obtaining mule varieties by art is one of the most important means that man possesses of modifying the works of nature, and of rendering them better adapted to his purposes. In our gardens some of the most beautiful flowers have such an origin; as, for instance, the roses obtained between R. indica and moschata, the different mule Potentillæ and Cacti, the splendid Azaleas raised between A. pontica and A. nudiflora coccinea, and the magnificent American-Indian Rhododendrons. By crossing varieties of the same species, the races of fruits and of culinary vegetables have been brought to a state as nearly approaching perfection as we can suppose possible. And if similar improvements have not taken place in a more important department,-namely, the trees that afford us timber, our experience fully warrants our entertaining the belief that, if proper means were adopted, improved varieties of as much consequence might be introduced into our forests, as have already been created for our gardens.

In conducting experiments of this kind, it is well to know that, in general, the characters of the female parent predominate in the flowers and parts of fructification; while the foliage and general constitution are chiefly those of the male parent. Thus, in the celebrated mule Rhododendron, gained by Lord Carnarvon by fertilising R. arboreum with R. Catawbiense, the mule variety had the flowers and colour of R. arboreum, but more the leaves and hardiness of constitution of R. Catawbiense.