NOVEMBER.

SECTION I.

SCIENCE OF GARDENING.

SCIENTIFIC OPERATIONS OF GARDENING.

THESE Operations include all the mechanical or artificial processes, by which plants are propagated, reared, and trained;—those by which their fertility is increased, accelerated, or retarded;-and, finally, those chemical processes, by which the food of plants is prepared, altered, or distributed. In the preceding sections, much has been already said on the laboration and distribution of vegetable aliment. The present section will, therefore, be devoted to a consideration of the operations connected with the arts of propagation and culture.

PART I.

OPERATIONS OF PROPAGATION.

1. BY SEED.

600. Quality of the Seed.-In order to secure a perfect plant, one that, in every respect, corresponds with the essential characters which distinguish the individual-the seed must be true to its kind, and free from admixture or spurious impregnation. It is a fact, the truth of which, the experience of ages, and that of every observant gardener, tends fully to establish, that congenerous plants are exceedingly liable to become crossed by intermixture; insomuch, that it may be laid down as a sound practical rule-never, in the same year, to attempt to procure perfect seeds from two or more plants of the same tribe, in any garden, be its dimensions what they may, unless the flowers of one of the plants have fallen or withered before those of the others appear.

601. Inquiry into the cause of intermixture.-The reader is

referred to what has been already advanced on the subject of impregnation by means of the pollen or farina, at No. 252. To these remarks, I add the following observations of Keith, on the authority of Du-Hamel, and other philosophers :

"The individual particles of the pollen are themselves organized substances, as may be seen by a good microscope, each particle consisting of a thin and membranous bag, capable of resisting the action of air, but extremely susceptible to the action of moisture, which as soon as it meets with, it explodes, like the anther itself, discharging a fine and subtile vapour, or a sort of fluid, in which there are contained globules still smaller.

"Gartner describes the globules of pollen as consisting of the following parts:-1st. An external cuticle, sometimes smooth, and sometimes set with hairs;-2nd. A cellular substance ;-3rd. A parenchyma, contained in the cells, and seemingly a rude and unorganized mass of granular matter. The globules, if put into water, swell and burst; first the cuticle, then the interior cells, and then the parenchyma, exploding and emitting a subtile and elastic vapour, or sort of fovilla, which swims on the surface. But the phenomenon does not take place in oil.

“Koelreuter describes each globule as consisting of two distinct membranes, an outer and an inner membrane, containing a cellular mass, from which a thin, oily, and inflammable fluid slowly exudes when placed in water, forming a shining and conspicuous pellicle that floats on the surface."

602. Whatever may be the internal structure of the farina, its powers of impregnation appear to depend solely upon specific affinities, acting through the medium of the membrane, which forms its exterior envelope. Thus, when the particles alight upon appropriate organs, such as the stigmas of congenerous plants, they affect those specific impregnations, which, in many tribes-particularly in that of brassica-are productive of endless crossings, re-crossings, and of a corresponding number of varieties.

Such plants as have few congeners-as the parsnep, carrot, rhubarb, asparagus, sea-kale, kidney-bean, artichoke, &c.—may be safely cultivated for raising seed; but great caution is required in all cases where plants abound in varieties. But, independently of the danger of intermixture, there is another point which merits much attention. It is this:-Seeds ripened year after year, in the same soil and situation, appear to deteriorate; they produce plants of inferior quality; and for this reason the farmer purchases his seedcorn in quarters far remote from those wherein he intends to cultivate his crops. From whatever cause, therefore, this deterioration

may proceed, it will be prudent not to raise seed of any kind, year after year, from the same stock, but to purchase or exchange seeds of annual and biennial plants every other year, in order to keep up a supply of perfect vegetables.

603. Agency of the pollen in the work of impregnation.-While speaking of the subject of intermixture, it will be interesting to inquire further into the philosophy of impregnation by the agency of the pollen, or farina fructicans. On this curious inquiry, the reader is referred to the sixth chapter of KEITH'S Physiological Botany, vol. ii., page 297. I now extract from the same work, the substance of the following observations, in order to furnish a concise view of the several theories of fecundation that have been advanced by some eminent naturalists. Of these, the principal

are:

(1.) Theory of the ovarist.-The term is to be traced to the Latin word ovum, an egg; and the theory supposes that "the embryo pre-exists in the ovary, and is fecundated by the agency of the pollen, as transmitted to it through the style." The theory appears to have been supported by Grew, Bonnet, Gærtner, and Haller; but has been most clearly elucidated by Spallanzani, an eminent Italian naturalist, who died in 1798. He chose for the subject of his observations, the Spanish broom--Spartium junceum-and commenced them when the flower-buds were not expanded. "When the pistil was freed from the surrounding integuments, and attentively viewed with a good glass, the pod was discovered of about one-tenth line in length. Several protuberances were seen upon its sides, which, upon opening it longitudinally, were found to be occasioned by seeds, which, though but small globules, were already discoverable, arranged in their natural order, and attached by filaments, to the interior of the pod. Upon dissection, they did not exhibit any appearance of the several parts and membranes into which the mature seed may be divided, but a spongy homogeneous mass. Flowers in the same state of forwardness were not fully expanded, till twenty days after. On dissecting buds of a larger size, the petals were found to be somewhat yellowish, and less compact; and the powder of the anthers was thrown out by the slightest agitation; but the lobes" (cotyledons) "and plantlet were not yet perceptible in the seeds.

"On the eleventh day after the flowers had fallen—that is, after impregnation had taken place—the seeds, which were formerly globular, began to assume the figure of a heart, attached to the pod by the basis, and exhibiting the appearance of a white point towards the apex; and when the heart was cut open longitudinally,

the white point proved to be a small cavity enclosing a drop of liquid.

"In forty days after the flower had fallen, the cavity was quite filled up with a body that had been generated within it, and which was now found to consist of a thin and tender membrane enveloping the two seed-lobes, between which the plantlet attached to the lower extremity was also perceptible. And hence, the seed was now visibly complete in all its parts."

(2.) Theory of the Animalculist.-This was founded upon the animal theory of Leuwenhoeck, and transferred to the generative production of vegetables by Morland, Needham, Gleichen, and others, who regard the pollen as being a congeries of seminal plants, one of which at least must be conveyed to the ovary entire, before it can become prolific.

Spallanzani could not detect any appearance of an embryo in the pollen, even by the highest magnifying powers; and, moreover, the well-known fact that "the total want of a passage in most styles, fit to conduct the particles of pollen entire, exposes this theory to the most serious objections. If the embryo is to pre-exist at all, is it not more likely that it should pre-exist in the ovary, where it is to be brought to maturity, than that it should first be generated in one organ or plant, and then transferred to another, to be developed?"-(KEITH, Vol. ii. p. 362.)

(3.) Theory of the Epigenesist.-The term is derived from the Greek words eπ, epi, which expresses contact, continuity, and co-existence; and yeveσis, genesis, a creation or production :-and the theory maintains that "the embryo pre-exists neither in the ovary nor in the pollen, but is generated by the union of the fecundating principles of the male and female organs, the former being the fluid issuing from the pollen when it explodes; and the latter, the fluid that exudes from the surface of the stigma when mature." This theory is supported by Koelreuter, who adduces in particular the result of his experiment on two species of the tobaccoplant-Nicotiana rustica and paniculata. "A flower of the former species was deprived of all its stamina, and fecundated with pollen from a plant of the latter species. The plant raised from the seed thus obtained, was a hybrid, exhibiting in all its parts an intermediate character betwixt the two species from which it sprang. The stamens of this hybrid, as well as of all others he ever raised, were imperfect; but when its pistils were impregnated with pollen from the paniculata, as before, the new hybrid obtained from the seeds now produced was more like a paniculata than formerly; and when the experiment was continued through several successive

generations, it was at last converted into a perfect paniculata.”— (KEITH, Vol. ii. 365.)

604. The electrical theory appears to me to combine, and thus to substantiate, the hypothesis of the ovarist and the epigenesist for it presumes, that as the integument or external membrane of the seed, and its homogeneous pulp, are found to pre-exist in the unimpregnated ovary, the attraction exerted between the fluids of the pollen and those of the stigma, through the medium of the membranous covering of the pollen when it alights on the stigma, produces those specific combinations which fertilize the heretofore passive ovary, and induce the formation and developement of the embryo and plantlet. Some interesting facts connected with the subject, and leading to practical utility, remain to be mentioned: they prove to a demonstration, the active agency of the farina.

605. Mr. Knight's celebrated experiment on the Pea.-Mr. Knight, in the year 1787, had a degenerate sort of pea growing in his garden, which was not restorable by attention and culture. "Being thus a good subject of experiment, the male organs of a dozen of its immature blossoms were destroyed, and the female organs left entire. When the blossoms had attained their mature state, the pollen of a very large and luxuriant gray pea was introduced into the one half of them, but not into the other. The pods of both grew equally; but the seeds of the half that was unimpregnated withered away, without having augmented beyond the size to which they had attained before the blossoms expanded. The seeds of the other half were augmented and matured as in the ordinary process of impregnation, and exhibited no perceptible difference from those of other plants of the same variety-perhaps because the external covering of the seed was furnished entirely by the female. But when they were made to vegetate in the succeeding spring, the effect of the experiment was obvious. The plants rose with great luxuriance, indicating in their stem, leaves, and fruit, the influence of this artificial impregnation: the seeds produced were of a dark gray. By impregnating the flowers of this variety with the pollen of others, the colour was again changed, and new varieties obtained, superior in every respect to the original on which the experiment was first made, and attaining in some cases to a height of more than twelve feet."-(Phil. Trans. 1789.)

606. In experiments upon the apple-tree Mr. Knight's success was also decisive. (See No. 38, page 44.) "The plants which were obtained in this case were found to possess the good qualities of both varieties employed, uniting the greatest health and luxuriance, with the finest and best flavoured fruit." (Phil. Trans. 1799.)