THE NATURE OF SEXUALITV. 903 



new plants without fertilisation and without any difference in the mode of their germination 

 from that of true oospores, with this single exception, that the quiescent period of the 

 parthenogenetic cells is shorter. 



Pringsheim's account of the development of the parthenogenetic forms of these 

 plants which is appended here is of great interest : — ' The successive generations both 

 of Saprolegnia ferax and of Achlya polyandra produced by cultivation become smaller, 

 and the number of male filaments diminishes in each succeeding generation until they 

 finally cease to be formed, and thus the monoecious forms become replaced by purely 

 female ones.' These observations show that as the result of continued cultivation combined 

 with the action of certain unfavourable conditions which accompany every attempt at 

 cultivation, the formation of male sexual organs at length ceases. Possibly it is in 

 consequence of the action of similar adverse conditions upon its internal constitution 

 that Chara cr'mita ceases to form antheridia after having grown for a considerable time 

 in certain waters. Possibly also these internal disturbances may affect the nature of the 

 female cells, though they are developed in the usual external form, so that they are not 

 sexually differentiated or only imperfectly so. In this case the effect would be one of 

 the obliteration of the existing sexual differentiation, or in other words, a case of 

 retrograde metamorphosis, and this is quite as conceivable as the first origin and the subse- 

 quent development of sexuality. In future investigations of the subject attention ought 

 to be paid to the question whether the oospheres developed upon plants of Saprolegnia 

 ferax and of Achlya polyandra which bear antheridia also are capable of parthenogenetic 

 development, or whether this property belongs only to oospheres developed by plants 

 destitute of antheridia ^. However difficult it may be to answer this question by experi- 

 ment, it must be done before it can be possible to decide whether or not the development 

 of the male organs deprives the oospheres of their power of independent development, so 

 that in proportion as the development of the male element diminishes the partheno- 

 genetic property increases. Since we may assume that the essential object of fertilisation 

 is to give to the oosphere something which it lacks but which is necessary for its further 

 development, a parthenogenetic oosphere must possess, independently of fertilisation, that 

 which it requires for its further development, that is, it is not sexually differentiated, and 

 this probably because the differentiation of the male element has been suppressed. 



(c) The Effects of Sexual Coalescence. Since nearly all plants, and more especially 

 the majority of Thallophytes, are capable of reproducing themselves asexually, and since 

 this' is the usual mode of reproduction in many species, it may well be asked what the 

 significance of sexual reproduction really is. If sexuality is merely concerned in the 

 development of new individuals, it is difficult to understand why the asexual reproduction 

 should not suffice. This question is of especial interest with reference to those lower 

 forms of Thallophytes which reproduce themselves through many generations by asexual 

 cells which may be either motile or non-motile : with reference to many Phanerogams, 

 for instance, the Conifers, it seems as if without sexuality, which induces the formation 

 of the seed, no reproduction would be possible. 



The significance of sexuality is seen in quite another light when those plants are 

 considered which exhibit a distinct alternation of generations, such as the Ferns, 

 Equisetaceae, the Mosses, and others. In Sect. 29 of Book I, I endeavoured to show 

 that the alternation of generations, wherever it occurs in the Vegetable Kingdom, 

 is produced by sexuality, and that without it no such alternation is possible. In all 

 cases of well-marked alternation of generations an organism which finally bears sexual 

 organs is developed from a spore which has been produced asexually; fertilisation 

 initiates a new process of development which closes with the development of the spore. 

 Before fertilisation, there is merely the organism developed from the spore, the first 

 or sexual generation : after fertilisation the second or asexual generation is developed 



^ [According to de Bary's recent investigations, the former of these two answers to the question 

 is the correct one.] 



