Book I. 



IMPREGNATION OF THE SEED. 



249 



without the previous agency of the pollen ; that the doctrine of the sexes of plants is consequently founded 

 i n fact. 



1591. Proofs from the economy of the aquatics. Many 

 plants of this class that vegetate for the most jwrt wholly 

 immersed in water, and often at a considerable dei)th, 

 gradually begin to elevate their stems as the season of 

 flowering advances, when they at last rear their heads 

 above the surface of the water, and present their opening 

 blossoms to the sun, till the petals have begun to fade, 

 when they agiin gradually sink down to the bottom to 

 ripen and to sow their seeds. This vei-y peculiar economy 

 may be exemplified in the case of ruppla maritlma, and 

 several species of potamogeton, common in our ponds 

 and ditches; from which we may fairly infer, that the 

 flowers rise thus to the surface merely to give the pollen 

 an opportunity of reaching the stigma uninjured. But 

 the most remarkable example of this kind is that of the 

 vallsnerla spiralis {fig. 238.), a plant that grows in the 

 ditches of Italy. The plant Is of the class Bicecia, pro- 

 ducing its fertile flowers on the extremity of a long and 

 slender stalk (a) twisted spirally like a corkscrew, which 

 uncoiling of its own accord, about the time of the open- 

 ing of the blossom, elevates the flowers to the surface of 

 the water, and leaves them to expand in the open air. 

 The barren flowers [jb) are produced in great numbers upon 

 short upright stalks Issuing from a different root, from 

 which they detach themselves about the time of the 

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

 reach the surface, 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 con- 

 tact, and giving the anthers an opportunity of discharging their pollen 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 its fruit at the bottom of 

 the water. . We have gathered (in 1819) these stalks, in the canals near Padua, upwards of ten feet long. 



Sect. VII. 



Impregnation of the Seed. 



1592. The stamens and pistils are the male and female organs of vegetable generation, and 

 the 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 ? 



1593. 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 infaUibly 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 In the discovery of honey, in quest of 

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

 covers its body with pollen, which It conveys totiie next flower It visits, and brushes oft" as It acquired it by 

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

 thus ettected. Nor is this altogether so nuich a work of random as It at first appears. For it has been 

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

 yet very often remain during the whole day upon the sj)ecles 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. 



1594. 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 tlie respective appellations of 

 ovarists, animalculists, and epigenesists. 



1595. 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. 



1596. Anitnalculid. 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-exlstence of anlmalcula in the seminal principle of the male ; the 

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

 developement. 



LW?. 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 

 prlncl()les 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 Dioecia ought occasionally to produce seeds whose offspring shall be Her- 

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



1598. 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 liom which they spring. Tliis 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. 



1599. 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 diseasetl 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 exixsrlments upon the common j)ea, 

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



