THE FERTILITY OF HYBRIDS. 579 



are 



either at the beginning or at the end of the period of bloom. Such hybrids 

 dependent upon foreign pollen, and if none is brought at the right time by wind or 

 insects no pollination or fertilization takes place, and no seeds are developed. 



Even these brief references to recently-discovered phenomena connected with 

 fertilization are sufficient to indicate that the suppression of the function of fruit- 

 formation in hybrids is due in most cases to the same causes as operate on true 

 species. Innumerable experiments have proved that if at the proper time pollen of 

 the right sort is placed upon the stigmas of hybrids fertile seeds are developed as in 

 the case of true species. 



We must now consider a statement which for long held its place in Botanical 

 works owing to the positive manner in which it was enunciated by an eminent 

 authority. The proposition in question, whilst admitting the fertility of hybrids, 

 asserted that it was conditional on the stigmas being supplied with pollen from one 

 or other of the parent-stocks, and that no fruit was formed as a result of autogamy. 

 This assumption rested partly on certain series of experiments performed on garden- 

 plants by the Botanist Koelreuter in the second half of the eighteenth century. 1 

 By crossing two species of Tobacco-plant (Nicotiana rustica and Nicotiana pani- 

 culata) Koelreuter produced a hybrid which in its characteristics was an exact 

 mean between the two parent-species. The stigmas in the flowers of this hybrid 

 were then dusted with pollen from one of the parents, and the result of this second 

 cross was another hybrid, the characters of which resembled those of the species 

 which supplied the pollen more closely than was the case with the first hybrid. The 

 same treatment was applied to the second hybrid, and thus, after three generations, 

 a plant was evolved completely resembling the male progenitor. The first hybrid 

 had, therefore, in a sense, "reverted" to that particular parent-species. The "rever- 

 sion " of the hybrid to the other parent-species was similarly procured after three 

 generations. Such a result could naturally not have ensued if the action of the 

 pollen of the parental stocks on the hybrid had not been entirely effectual. It is 

 accordingly quite true that hybrids are fertile when the pollen used to fertilize 

 them is taken from either of the parent-species, but the further assertion that they 

 are sterile if their own pollen is employed for the purpose is incorrect, at any rate 

 as a universal proposition. Koelreuter's own conscientious experiments show con- 

 clusively that it is possible for hybrids to bring fruits to maturity as a result of 

 autogamy, and that, as a matter of fact, the majority do develop such fruits. We 

 may also refer to the large number of plants with ornamental flowers, such as 

 Begonias, Pansies, and Pinks, which every year in our gardens produce seeds auto- 

 gamously, and are reproduced in great numbers by means of those seeds (see p. 556). 

 Some interesting experiments have also been made on Medicago media, the hybrid 

 of Medicago falcata and M. sativa. This plant, which is, in many places, cultivated 

 on a large scale for fodder, is propagated continuously by seeds which are in a very 



1 Joseph Gottlieb Koelreuter (1733-1806) was the first to investigate the question of hybridization scientifically 

 and thoroughly. His work, which ranks with the best of modern times, is contained in his VorltLufige Nachricht 

 von einigen das Geschhcht der Pftamen betreffenden Versucken und Beobachtungen (published 1761-1766) ; a convenient 

 reprint was issued in 1893. 



