embryo, as already described, easily establishes the analogy of Phanerogamous plants and those Cryptogamic plants in which the spores are evident conversions of the cellular tissue of the foliaceous organs or leafy expansions; for the same part furnishes the groundwork of a new plant in both groups, and the only difference existing between the two is this; in Phanerogamæ a previous formative process in the interior of the plant precedes a period of latent vegetation, whilst in Cryptogama the spore (the grain of pollen) developes itself as a plant without previous preparation. Difficulties nevertheless occur here in the consideration of Mosses and Hepaticæ, and more particularly in the enigmatical Marsileaceæ. It appears to me, however, that in this last-named family especially, there still remains much to be observed."

The opinion of Endlicher is to a certain extent that of Schleiden; that is to say, he considers what we call pollen analogous to the spores of Cryptogamic plants, and consequently the anther a female organ, whose contents perform an act similar to that of germination, when they fall upon the stigma; he does not, however, with Schleiden, assign a male influence to the sac of the amnios, but he attributes that property to the stigmatic papillæ, whose moisture lubricates the grains of pollen when they fall upon them.* I know of no one else who maintains this last opinion; but it deserves to be noted that Morren observed a circulating movement (he calls it cyclosis) in the fluid filling the papillæ of Cereus grandiflorus at the period of impregnation.

One of the most curious consequences of the presence of

* See Grundzüge einer neuen Theorie der Pflanzenzeigung. Professor Wydler of Berne, also, insists upon the pollen being the female apparatus, and he denies that plants have two sexes. (Recherches sur l'Ovule, &c., des Scrofulaires.) These speculations have all arisen out of the undoubted fact, that the developement of spores and pollen grains takes place in the same manner, and that there is considerable resemblance in their final structure. This was, I think, first noticed by Mohl (Ueber die Entwicklung der Sporen, &c.), in 1833; Mirbel, in 1835, stated that there was a marvellous resemblance between these parts (Ann. Sc., n. s., iv. 9.); Morren declares that the spore is organised like a grain of pollen (Anat. des Jungermann. p. 10.); and, finally, Wydler admits a great analogy between the formation of pollen and the spores of many foliaceous cryptogamic plants.

sexes in plants is, the property the latter consequently possess of producing mules. 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 permanently by seed; should it not be absolutely sterile, it will become so after a few generations. 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 abundant facility, there being a great accordance between them in general structure and constitution; but no one has ever succeeded in compelling the Pear to fertilise the Apple, or 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, in general present an insuperable bar to the intercourse of different genera. All the stories that are current as to the intermixture of Oranges and Pome

granates, of Roses and Black Currants, and the like, may, therefore be set down to pure invention.

It is, nevertheless, apparently true, that bigeners, that is to say, mules between different genera, have in some few cases been artificially obtained. Kölreuter obtained such between Malvaceous plants; Gærtner, between Daturas and Henbane and Tobacco; Wiegman, between a Garden Bean and a Lentil; and there are other well-attested cases. But all such productions were as short-lived and sickly as they were monstrous.

As this power of creating mule plants fertile for two or three generations incontestably exists, it is not to be wondered at, that in wild nature hybrid varieties should be far from uncommon. 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 Bentham. The same acute botanist ascertained that Saxifraga luteopurpurea of Lapeyrouse, and S. ambigua of De Candolle, are only wild accidental hybrids between S. aretioides and calyciflora : they are only found where 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; and altogether about forty cases of wild reputed species of the genera Ranunculus, Anemone, Hypericum, Scleranthus, Drosera, Potentilla, Geum, Medicago, Galium, Centaurea, Stachys, Rhinanthus, Digitalis, Verbascum, Gentiana, Mentha, Quercus, Salix, and Narcissus, have been collected by Schiede, Lasch, and De Candolle; to which far too many may be added from the works of species-making botanists. It is impossible not to believe that a great proportion of the reputed species of Rosa, Rubus, and other intricate genera, have had a hybrid origin.

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, experience fully warrants 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.

It is, however, to be regretted that those who occupy themselves with experiments of this kind do not confine them to woody or perennial plants which can be perpetuated by cuttings. Mule annuals have the great fault of perishing almost as soon as they are obtained, and they serve no other purpose than that of encumbering the records of science with accounts of so called species which, from their transitory existence, can never be re-examined.

These, however, are considerations which belong to Horticulture rather than to Botany. The reader who would make himself acquainted with the practical bearing of the subject should study Mr. Herbert's work on Amaryllidaceæ, p. 335. to 380.

The cause of the frequent sterility of mule plants is at present 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 mule 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 pistil, 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.

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CHAPTER IX.

OF THE FRUIT.

THE fruit is mechanically destined as a mere protection to the seed; it constitutes the principal part of the food, especially in winter, of birds and small animals; it is often more ornamental than the flowers themselves, and it contributes most materially to the necessities and luxuries of mankind. When ripe, it falls from the plant, and, borne down by its weight, lies on the ground at the foot of the individual that produced it: here its seeds vegetate, when it decays, and a crop of new individuals arises from the base of the old one. But, as plants produced in such a manner would soon choke and destroy each other, nature has provided a multitude of ways for their dispersion. Many are carried to distant spots by the animals which eat them: others, such as the samara, and the pappus of Compositæ, provided with a sort of wing, fly away upon the wind to seek a distant station; others scatter their seeds abroad by an explosion of the pericarp, caused by a sudden contraction of the tissue; many, falling upon the surface of streams, are carried along by the current; while others are dispersed by a variety of methods which it would be tedious

to enumerate.

The fruit, during its growth, is supported at the expense of the sap generally: but most especially of that which had been previously accumulated for its maintenance. This is less apparent in perennial or ligneous plants than in annual ones, but is capable of demonstration in both. Knight has well observed, that in annual fruit-bearing plants, such as the Melon, if a fruit is allowed to form at a very early period of the life of the plant, as, for instance, in the axil of the third leaf, it rarely sets or arrives at maturity, but falls off