404 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Hence we may define fertilization as the fusion of two representative 

 sexual cells or gametes, and the production therefrom of a single cell, 

 the zygote, which is an incipient individual. 



Since the female gamete, or egg-cell of an ovule, does not, as a rule, 

 form an embryo unless it encounters and fuses with a male gamete, it 

 is evident that this fusion is the essential part of fertilization, and that 

 pollination and the growth of the pollen tubes are but the necessary 

 means to that end. One of the most striking facts which emerge from 

 these discoveries is that both parents make material contributions to 

 the substance of the offspring. 



The nuclear contributions appear to be equal in amount and 

 generally similar in kind, but the contributions of cytoplasm made by 

 the two gametes or sexual cells appear to be unequal : the cytoplasm of 

 the egg-cell being considerable ; that of the male gamete being smaller 

 in amount and in some cases nil. In the similarity between the nuclei 

 we have the explanation of the well-known fact that, of the many 

 characters exhibited by the offspring, some are derived from one parent, 

 others from the other parent, and others again are intermediate between 

 the paternal and maternal characters. We are not yet, however, in a 

 position to assert that all the characters of the offspring are contributed 

 in equal measure by the two parents. 



On this subject, however, despite its interest and importance, we 

 cannot dwell now ; for we must continue our sketch of the phenomena 

 of fertilization. Inasmuch as the normal egg-cell does not develop 

 unless it has united with the male gamete, it follows that the latter 

 not only contributes living material toward the upbuilding of the future 

 plant, but that the male cell alsO' provokes or stimulates the egg-cell 

 to grow into the embryo. It might be supposed, and indeed it was 

 supposed until recently, that the growth of the fertilized egg-cell is the 

 direct consequence of the yoking together of the male and female 

 gametes. Just as one horse alone might not be able to draw a cart 

 which two horses may pull easily, so it might be imagined that the 

 female or egg cell alone is unable to carry on the work of development, 

 which work is easy to the combined forces of the male and female 

 gametes. Eecent experiments have shown that this supposition is not 

 correct, and that the development of the egg-cell to form the embryo 

 is due to a definite shock or stfmulus which it receives from the male 

 gamete. In terms of our simile, under the powerful stimulus of the 

 whip the single horse may make a supreme effort and set the cart 

 in motion. We are, therefore, obliged to recognize that fertilization has 

 a two-fold significance. On the one hand it allows of the formation of 

 an individual which derives materially from the father and the mother, 

 and hence may exhibit paternal and maternal characters ; on the other 

 hand, the male gamete in entering into the egg-cell exerts a stimulus 

 which starts the latter on a course of growth and division resulting in 

 the formation of the embryo. 



The evidence in support of the latter conclusion is clear. Thus it 

 has been shown in certain animals and plants that the fertilization 



