250 SCIENCE OF AGRICULTURE. Part II. 



expansion of the female blossom, mounting up like little air bubbles, and suddenly expanding when they 

 reach the surface (c), where they float about in great numbers among the female blossoms, and often cling 

 to them in clusters, so as to cover them entirely ; thus bringing the stamens and pistils into immediate 

 contact, and giving the anthers an opportunity of discharging their puUen immediately over the stigma. 

 "When this operation has been performed, the now uncoiled stalk of the female plant begins again to resume 

 its original and spiral form, and gradually sinks down, as it gradually rose, to ripen it's fruit at the bottom 

 of the water. In 1819, we gathered these stalks, in the canals near Padua, upwards of ten feet long. 



Sect. VII. Impregnation of the Seed. 

 1625. The stamens and pistils are the male and female organs of vegetable generation, and 

 the pollen is the substance bi/ 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 ? 



1626. Access of the pollen. When the stamens and pistils are situated near each other, the elastic spring 

 with wliich the anther flies open, will generally be suflScient 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 eflected. 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 si)ecies 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. 



1627. Fecundation of the ovary. Admitting that the pollen is conducted to the ovary 

 through the tubes of the style, how after all is the ovary fecundated, or the seed ren- 

 dered fertile ? On this subject naturalists have been much divided ; and, according to 

 their several opinions, have been classed under the respective appellations of ovarists, 

 animalculists, and epigenesists. 



1628. Ovarist. According to the opinion of the Ovarist, the embryo preexists in the ovary, and is 

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



1629. 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 preexistence of animalcula in the seminal principle of the male ; the 

 animalcula being conveyed in co'itu to the ovary of the female, where alone they are capable of 

 developement. 



1630. 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 preexists neither in the ovary nor pollen, but is generated by the union of the fecundat- 

 ing 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. 



1631. Hybrids. Although the arguments of the epigenesists are by no means satis- 

 factory, yet it cannot be denied, that hybrid productions partake 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. 



1632. 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 ardour of his desire to obtain 

 information 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 colour which italTorded. In 1787, adegenerate sort of pea was growing in his garden, which had 

 not recovered its former vigour even when removed to a better soil. Being thus a good subject of ex- 

 periment, 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 eflfiect of 

 the experiment was obvious. The plants rose with great luxurance, 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 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.) Knight thinks his 

 experiments on this subject afford examples of superfcetation, a phenomenon, the existence of which 

 appears doubtful amongst animals, and of which the proof amongst vegetables is not yet quite satisfactory. 

 Of one species of superfcetation 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 obtamed. 

 But of the other species of superfcetation, 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 curiosity, and not apparently con- 

 nected with any views of utility. 



1633. The practicability of improving the species is rendered strikingly obvious by these experiments ; 

 and the ameliorating efl^^ect 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- 

 periments 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 



