904 PHENOMENA OF SEXUAL REPRODUCTION. 



M'hich bears spores. If we compare the histological and the morphological development 

 : of the two generations, it becomes strikingly evident, among the Vascular Cryptogams 

 at least, that the generation which has been developed as the result of fertilisation 

 is much more highly organised than the generation (prothallium) which has been 

 developed from the spore. In the Mosses it might appear that the contrary is the 

 ^ case, for in these plants the sexual generation is the one which grows independently and 

 which is differentiated into leaf and stem ; however, the histological differentiation of the 

 sporogonium is far more perfect than that of the moss-plant, so that it is true for Mosses 

 also that the product of fertilisation is the more highly organised of the two generations. 

 All these cases of evident alternation of generations lead to the conclusion that 

 a process of development of a more complex kind is initiated by the sexual act. 

 To a certain extent this is true also of the Zygomycetes, the zygospore being a more 

 highly organised cell than any of those of the mycelium, and it can scarcely be doubted 

 with reference to the Coleochaeteae, the Gharaceae, and the Florideae that the sexually- 

 formed spore-fruit is histologically the most complex product of these plants. Although 

 the same cannot be said with reference to the Oosporese and to the Conjugatae, still this 

 by no means affects the significance of the sexual act for other plants. A complete 

 discussion of the facts would, on the contrary, probably show that the higher develop- 

 ment of the Pandorineae, of the Conjugatae, and of the Diatomaceae when compared with 

 the Protophyta has been probably promoted by the evolution of sexuality, even though 

 this is not expressed by a well-marked alternation of generations. The Phanerogams 

 afford a similar, though exactly opposite case : in them the alternation of generations is 

 exhibited in only a rudimentary form, for, in the course of the development of these 

 plants from some primitive type allied to the Vascular Cryptogams the sexual generation 

 (prothallium) has been reduced to its simplest expression. Whereas in the Oosporeae 

 the sexual generation is the predominating one and the product of fertilisation is but 

 imperfectly developed, in Phanerogams it is the generation produced in consequence 

 of a sexual act which comes to be completely developed, and it is the sexual generation 

 (Prothallium, Endosperm) which is rudimentary. In the latter case we have the end, in 

 the former the beginning of phytogenetic series ; in the latter the alternation of genera- 

 tions is disappearing, in the former it is in the first stage of its evolution. If therefore 

 we desire to understand the significance of sexuality in the history of the development 

 of a single plant or in that of the whole Vegetable Kingdom, we must fix our attention 

 upon those groups in which an alternation of generation is evident: in such 'cases 

 (Vascular Cryptogams, Muscineae) the effect of sexuality is obvious. We may then con- 

 clude that the coalescence of the male with the female cell causes the development of an 

 organism which is more highly differentiated both histologically and morphologically. 



Sect. 33. 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 thus be more or less nearly related to one another as 

 having been 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 preservation 

 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 Rhynchone7na 

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



