J82 SCIENCE OF GARDENING. Part II. 



Sect. VII. Impregnation of the Seed. t 



814. The stamens and pistils are the male and female organs of vegetable generation, and 

 tlie pollen is the substance by which the impregnation of the seed is effected ; but how is the 

 pollen conveyed to the ovary ? And what is the amount of its action ? 



815. Access of the pollen. When the stamens and pistils are situated near each other, the elastic spring 

 with which the anther flies open will generally be sufficient to disperse the pollen, so as that part of it 

 must infallibly reach the stigma in such flowers as do not perfect their stamens and pistils at the same 

 time. The pollen is very generally conveyed from the anther to the stigma through the instrumentality of 

 bees, and other insects peculiar to a species. The object of the insect is the discovery of honey, in quest of 

 which, whilst it roves from flower to flower, and rummages the recesses of the corolla, it unintentionally 

 covers its body with pollen, which it conveys to the next flower it visits, and brushes off' as it acquired it by 

 rummaging for honey ; so that part of it is almost unavoidably deposited on the stigma, and impregnation 

 thus effected. Nor is this altogether so much a work of random as it at first appears. For it has been 

 observed that even insects, which do not upon the whole confine themselves to one species of flower, will 

 yet very often remain during the whole day upon the species they happen first to alight on in the morning ; 

 hence the impregnation of the females of Dioecious plants where no male is near. Hence also a sort of 

 natural crossing of the breed of plants which might probably otherwise degenerate. 



816. Fecundation of the ovary. Admitting that the pollen is conducted to the ovary 

 through the channel of the tubes of the style, how after all is the ovary fecundated ; or 

 the seed rendered fertile ? On this subject naturalists have been much divided ; and ac- 

 cording to their several opinions have been classed under the respective appellations of 

 ovarists, animalculists, and epigenesists, 



817. Ovarist. According to the opinion of the Ovarist, the embryo pre-exists in the ovary, and is 

 fecundated by the agency of the pollen as transmitted to it through the style. 



818. Animalculist. But the theory of the ovarists is not without its difficulties ; for as the embryo is 

 never found to make its appearance till after fecundation, it has been thought that it must necessarily pre- 

 exist in the pollen of the anther; from which it is conveyed to the ovary through the medium of the style, 

 and afterwards matured. This theory was founded upon that of Leuwenhoeck, with regard to animal 

 generation ; which supposes the pre-existence of animalcula in the seminal principle of the male ; the 

 animalcula being conveyed in coitu to the ovary of the female, where alone they are capable of 

 developsment. 



819. Epigenesist. The difficulties inseparable from both theories, together with the phenomenon of 

 hybrid productions, have given rise also to a third ; this is the Theory of the Epigenesists, who maintain that 

 the embryo pre-exists neither in the ovary nor 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. But if 

 the seed is generated from the union of two fecundating principles which form an intermediate offspring, 

 then female plants of the class Dicecia ought occasionally to produce seeds whose offspring shall be Her- 

 maphrodite, or at least Monoecious, which was never yet known to happen. 



820. Hybrids. Although the arguments of the epigenesists are by no means satis- 

 factory, yet it cannot be denied, that hybrid productionstpartake of the properties both of 

 the male and female from which they spring. This was long ago proved to be the fact 

 by Bradley, and more recently confirmed by the experiments of Knight ; as well as hap- 

 pily converted to the advantage of the cultivator. 



821. Vegetable crossing. Observing that farmers who rear cattle improve the progeny by means of crossing 

 the breed, Knight argued from analogy-, that the same improvement might be introduced into vegetables. 

 His principal object was that of procuring new and improved varieties of the apple and pear to supply the 

 place of such as had become diseased and unproductive. But as the necessary slowness of all experiments 

 of the kind, with regard to the fruit in question, did not keep pace with the ardor of his desire to obtain in- 

 formation on the subject, he was induced to institute some tentative experiments upon the common pea, 

 a plant well suited to his purpose, both from its quickness of growth, and from the many varieties in form, 

 size, and color, which it afforded. In 1787, a degenerate sort of pea was growing in his garden, which had 

 not recovered its former vigor even when removed to a better soil. 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 grey 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 were 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 grey. By im- 

 pregnating the flowers of this variety with the pollen of others, the color 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.) Knight thinks his 

 experiments on this subject afford examples of superfoetation, a phenomenon, the existenceof which has 

 been admitted amongst animals, but of which the proof amongst vegetables is not yet quite satisfactory 

 Of one species of superfoetation he has certainly produced examples ; that is, when, by impregnating a 

 white pea-blossom with the pollen both of a white and grey pea, white and grey seeds were obtained 

 But of the other species of superfoetation, in which one seed is supposed to be the joint issue of two males 

 the example is not quite satisfactory. Such a production is perhaps possible, and further experiments' 

 may probably ascertain the fact ; but it seems to be a matter of mere curiositv, and not apparently con 

 nected with any views of utility. 



822. The practicability of improving the species, is rendered strikingly obvious bv these experiments - 

 and the ameliorating effect is the same whether by the male or female ; as was ascertained by impreg- 

 nating the largest and most luxuriant plants with the pollen of the most diminutive and dwarfish or 

 the contrary. By such means any number of varieties may be obtained, according to the will of the 

 experimenter, amongst which some will no doubt be suited to all soils and situations Knight's ex 

 penments of this kind were extended also to wheat; but not with equal success. For though some 

 very good varieties were obtained, yet they were found not to be permanent. But the success of his 

 experiments on the apple-tree were equal to his hopes. This was indeed his principal object and no 

 means of obtaining a successful issue were left untried. The plants which were obtained in this eafe 



