LECTURE XLV. 



INFLUENCE OF THE ORIGIN OF THE SEXUAL CELLS OF THE 

 SAME SPECIES ON THE RESULT OF FERTILISATIONS 



The male and female cells, or the organs which produce them, arise either 

 close to one another or at a distance on the same plant, or they 'arise on 

 different individuals of the same species; the sexual cells of the same species of 

 plant may thus, according to their origin, be more or less closely related, 

 behaving towards one another as sister-cells, or as cousins, or as their grand- 

 children and great grand-children, and so on. The question now arises as to 

 what influence this relationship in the origin of the male and female cells exerts 

 on the result of fertilisation. At present it is true no general law can be 

 formulated in this connection, but by far the majority of the phenomena point 

 to the view that the sexual union of very closely related sexual cells is generally 

 avoided, and this the more the more advanced the morphological and sexual differ- 

 entiation. It is only in the case of a few of the lower plants that it happens 

 that the sexual cells which unite with fertility are sister-cells, e. g. in Rhynchonema 

 among the Conjugatae; even in most of the other Algae and Fungi the sexual 

 cells of the same plant are more distantly related (Spirogyra, (Edogonium, Fucus 



' The remarkable relations of insects to the fertilisation of flowers treated in this lecture were 

 first described at length by Christian Conrad Sprengel in his extremely remarkable and inspired 

 work, 'Das neu entdeckte Geheimniss der Natur im Bau und in der Befruchtung der Blumen ' 

 (with 25 copper plates, Berlin, 1793), after Jos. Gottlieb Koelreuter (1761) had already pointed out 

 in his ' Vorldufigen Nachrichten, das Geschlecht der PJlanzen belreffend,' the necessity of the aid of 

 insects in the pollination of many flowers. Sprengel indeed expressed the fruitful idea, "since very 

 many flowers have separate sexes, and probably at least as many hermaphrodite flowers are dicho- 

 gamous, Nature appears to be unwilling that any flower shall be fertilised by means of its own pollen.' 

 Sprengel's work remained unnoticed until about twenty years ago ; it was rescued from obscurity by 

 Charles Darwin, and the doctrine extended by new observations of the latter and incorporated with 

 the theory of descent. Stimulated by Darwin's work on the fertilisation of the Orchids (1862) and 

 by other works of the same author, an extensive literature on this subject has been developed, of 

 which I will only quote one or two titles. 



Friedrich Hildebrand, 'Die Geschlechtsvertheilung bet den PJlanzen und das Gesetz der vermie- 

 denen und unvortheilhaften stetigen Selbstbefruchiung' (Leipzig, 1S67). 



The most exhaustive and fundamental work on the facts of this subject is Hermann Miiller's 

 comprehensive work, 'Die Befruchtung der Blumen durch Insekten und die gegenseitigen An- 

 passungen beider'' (Leipzig, 1873). Translated into English by D'Arcy W. Thompson. 



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