INFLUENCE OF THE ORIGIN OF CELLS ON FERTILISATION. So/ 



Sect. 31. Influence of the origin of the reproductive cells on the 

 product of fertilisation. The male and female cells or the organs that produce 

 them are formed at a greater or lesser distance from one another on the same 

 plant, or on different individuals of the same species. The male and female cells 

 of the same species may also be more or less nearly related to one another as 

 immediately or more remotely derived from the same parent-cell. The question 

 arises what influence this genetic relationship of the male and female cells exercises 

 on the product of fertilisation. At present we are unable to lay down any general 

 law in this respect; but the overwhelming weight of evidence points to the law 

 that the sexual union of nearly related cells is detrimental to the propagation 

 of the plant, and in general the more so the further the morphological and sexual 

 differentiation of the species has advanced. Only in a few plants of low organ- 

 isation does a fertile union take place between sister-cells, as in Rhynchonema 

 among Conjugatse. But in most Algae and Fungi (as Spirogyra, CEdogonium, 

 Fucus platycarpiis, &c.) the reproductive cells of the same plant are not so closely 

 related, and especially where fertilisation is caused by actively or passively motile 

 spermatozoids, there being at least a possibility of their meeting with oospheres of 

 more remote origin. Even in Vaucheria, where the antheridium is the sister-cell of 

 the oogonium, the curving of the former, and the direction in which the spermatozoids 

 escape, indicates that fertilisation does not usually take place between the contiguous 

 organs, but between those more remote or even between those produced by different 

 individuals. The tendency for fertilisation to occur only between reproductive cells 

 of as remote relationship as possible within the same species is manifested in a great 

 variety of contrivances, the simplest being that on each individual of the sexual 

 generation only male or only female organs are produced. Between the two uniting 

 reproductive cells lies also the entire course of development of the two plants when 

 they are derived from the same mother- plant, and a still longer course of develop- 

 ment when they are derived from different mother-plants. This distribution of the 

 sexes, which is generally termed Dioecism, occurs in all classes and orders of the 

 vegetable kingdom, showing that it is a useful contrivance for the maintenance of 

 different species. Thus we find this phenomenon in many Algae as in most Fuca- 

 ceae, in some Saprolegnieae andCharacese {Nilella syticarpa,&:c.), in many Muscineae, 

 in the prothallium of many Ferns {Osmunda regalis) and of most Equisetaceae, and 

 in many Gymnosperms and Angiosperms. 



If the plant which produces both kinds of sexual organs is large or at least 

 strongly differentiated, distance in the relationship of the two kinds of reproductive 

 cells is still attained by the male and female organs being produced on different 

 branches; and this phenomenon, which is in general termed Moncecismy is also 

 common in the vegetable kingdom, as in some Algae, many Muscineae, and a very 

 large number of Gymnosperms and Angiosperms \ 



But another relationship which, according to the law just stated, should apparently 

 be very unfavourable, is also of very common occurrence in the vegetable kingdom, 

 — where the reproductive organs are in close contiguity, and the sexual cells therefore 



^ The arrangement of the reproductive organs termed Polygamy is also a contrivance intended 

 to hinder perpetual self-fertilisation of a flower or of an individual. 



