« PreviousContinue »
By what means the first tendency to change their nature was given to domesticated plants, we are entirely ignorant. It is probable that it was originally due to accident, and also that it was still mere chance which continued to operate down to very modern times. Philosophers are unacquainted with the reason why there should be any tendency to variation from the characters first stamped on any species by Nature; but all know that this tendency does exist, and in a most remarkable degree in many species. There is in all beings a disposition to deviate from their original nature when cultivated, or even in a wild state; but this disposition is so strong in some as to render them particularly well adapted to become subject to domestication : for instance, the dog, the pigeon, and the barn-yard fowl, are cases in which this tendency is most strongly marked in animals; and domesticated fruits are a parallel case in the vegetable world.
Without, then, vainly endeavouring to discover the first cause of this disposition to form varieties, let us take it as a naked fact that the disposition exists. Cultivators increase this disposition chiefly in two ways; either by constantly selecting the finest existing varieties for seed, or by intermixing the pollen and stigma of two varieties for the purpose of procuring something of an intermediate nature. The ancients were unacquainted with either of these practices, and consequently their gardens contained few things which would now be deemed worthy of cultivation. The power of obtaining cross-bred varieties at pleasure has only existed since the discovery of sexes in plants; but as it exerts a most extensive influence over alterations in the vegetable kingdom, it may be considered the most important controlling power that we possess.
In sowing seeds for the purpose of procuring improved varieties, care should be had, not only that the seeds be
taken from the finest existing kinds ; but also that the most handsome, the largest and the most perfectly ripened specimens should be those that supply the seed. A seedling plant will always partake more or less of the character of its parent, the qualities of which are concentrated in the embryo when it has arrived at full maturity. How this concentration takes place, we are as ignorant as why certain constitutional peculiarities are in men transferred from father to son, and from generation to generation ; but we know that it does take place. Now if the general qualities of a given variety are concentrated in the embryo. under any circumstances, it is reasonable to suppose that they will be most especially concentrated in a seed taken from that part of a tree in which its peculiar good qualities reside in the highest degree. For instance, in the fruit of an apple growing upon a north wall there is a smaller formation of sugar than in the same variety growing on a south wall ; and it can be easily understood that the seed of that fruit which is itself least capable of forming saccharine secretions, will acquire from its parent a less power of the same nature than if it had been formed within a fruit in which the saccharine principle was abundant. It should therefore be always an object with a gardener, in selecting a variety to become the parent of a new sort, to stimulate that variety by every means in his power to produce the largest and the most fully ripened fruit that it is capable of bearing. The importance of doing this is well known in regard to Melons and Cucumbers, and also in preserving fugitive varieties of flowers; but it is not generally practised in raising fruit trees.
The power of procuring intermediate varieties by the intermixture of the pollen and stigma of two different parents is, however, that which most deserves consideration. We all know that hybrid plants are constantly
produced in every garden, and that improvements of the most remarkable kind are yearly occurring in consequence. Experiments are, however, it may be supposed, sometimes made without the operator being exactly aware either of the precise nature of the action to which he is trusting for success, or of the limits within which his experiments should be confined.
Cross fertilisation is effected, as every one knows, by the action of the pollen of one plant upon the stigma of another. The nature of this action is highly curious. Pollen consists of extremely minute hollow balls or bodies; their cavity is filled with fluid, in which swim particles of a figure varying from spherical to oblong, and having an apparently spontaneous motion. The stigma is composed of very lax tissue, the intercellular passages of which have a greater diameter than the moving particles of the pollen.
When a grain of pollen comes in contact with the stigma, it bursts and discharges its contents among the lax tissue upon which it has fallen. The moving particles descend through the tissue of the style, until one, or sometimes more, of them finds its way, by routes specially destined by nature for this service, into a little opening in the integuments of the ovulum or young seed. Once deposited there, the particle swells, increases gradually in size, separates into radicle and cotyledons, and finally becomes the embryo, — that part which is to give birth, when the seed is sown, to a new individual.
Such being the mode in which the pollen influences the stigma and subsequently the seed, a practical consequence of great importance necessarily follows, viz. that in all cases of cross fertilisation the new variety will take chiefly after its polliniferous or male parent; and that at the same time it will acquire some of the constitu
tional peculiarities of its mother.* Thus, the male parent of the Downton Strawberry was the Old Black, the female a kind of Scarlet ; in Coe's Golden Drop Plum, the father was the Yellow Magnum Bonum, the mother the Green Gage ; and in the Elton Cherry the White Heart was the male parent, and the Graffion the female.
The limits within which experiments of this kind must be confined are, however, narrow.
It seems that cross fertilisation will not take place at all, or very rarely, between different species, unless these species are nearly related to each other; and that the offspring of the two distinct species is itself sterile, or if it possesses the power of multiplying itself by seed, its progeny returns back to the state of one or other of its parents. . Hence it seldom or never has happened that domesticated fruits have had such an origin. We have no varieties raised between the Apple and the Pear, or the Quince and the latter, or the Plum and Cherry, or the Gooseberry and the Currant. On the other hand, new varieties obtained by the intermixture of two pre-existing varieties are not less prolific, but, on the contrary, often more so than either of their parents; witness the numerous sorts of Flemish Pears which have been raised by cross fertilisation from bad bearers, within the last twenty years, and which are the most prolific fruit trees with which gardeners are acquainted ; witness also Mr. Knight's Cherries, raised between the May Duke and the Graffion, and the Coe's Plum already mentioned.
It is, therefore, to the intermixture of the most valuable existing varieties of fruit that gardeners should trust for the amelioration of their stock. By this operation the pears that are in eating in the spring have been
* In early crosses between distinct species this is particularly manifest ; but in those of varieties long domesticated it is less apparent, the distinctions between the parents themselves being less fixed, and less clearly marked.
rendered as delicious and as fertile as those of the autumn ; and there is no apparent reason why those very early, but worthless sorts, such as the Muscat Robert, which usher in the season of Pears, should not be brought to a similar state of perfection.
There is no kind of fruit, however delicious, that may not be deteriorated, or however worthless, that may not be ameliorated, by particular modes of management; so that after a given variety shall have been created, its merits may still be either elicited or destroyed by the cultivator. In this place those practices only need be considered that tend to improvement.
Some fruits of excellent quality are bad bearers : this defect is remedied by a variety of different methods, such as, 1. By ringing the bark ; 2. By bending branches downwards ; 3. By training ; and, 4. By the use of different kinds of stocks. All these practices are intended to produce exactly the same effect by different ways. Physiologists know that whatever tends to cause a rapid diffusion of the sap and secretions of any plant, causes also the formation of leaf buds instead of flower buds; and that whatever, on the contrary, tends to cause an accumulation of sap and secretions, has the effect of producing flower buds in abundance. This circumstance, which at first sight seems to be difficult to account for physiologically, is no doubt to be explained by the difference between leaf buds and flower buds themselves. In a leaf bud, all the appendages or leaves are in a high state of development, and the central part or axis, around which they are arranged, has a tendency to extend itself in the form of a branch as soon as the necessary stimulus has been communicated to the system by the light and warmth of spring. In a flower bud, the appendages or leaves are in that imperfectly formed, contracted state, which we name calyx, corolla, stamens, and pistilla ; and the central part