ESSENTIAL ELEMENT IN THE PROCESS OF SEXUAL REPRODUCTION. 805 



Algae, Characese, Muscinese, Ferns, and Equisetaceae, the nature of the alternation of 

 generations is such that the sexual differentiation is developed in one of the gener- 

 ations, while it is neutralised in the succeeding generation. In these cases therefore 

 we have a sexual and an asexual generation in the course of development of the 

 same individual ; the asexual generation is the product of the neutralising of the 

 sexual differentiation of the sexual generation. The two generations, especially in 

 IVIuscineae and Vascular Cryptogams, differ essentially from a morphological point 

 of view ; they follow altogether different laws of development ; one of their 

 limits always occurs in the fertilised oosphere. The prothallium developed from 

 the asexual spore of Ferns and Equisetaceae is, for example, morphologically a 

 thallus without leaves or roots, while its physiological significance is determined by 

 the production of antheridia and archegonia. From the fertilised oosphere on the 

 other hand is produced the Fern or Horsetail, characterised morphologically by the 

 differentiation of stem, root, and leaf ; but sexually this differentiated plant is neuter, 

 producing neither male nor female cells, but only asexual spores. If the process 

 of development of Rhizocarpese and Selaginelleae is compared with these pheno- 

 mena, it will be seen that in these classes the two generations, the prothallium and 

 the spore-forming leafy plant, stand essentially in the same relation to one another 

 as in Ferns and Equisetaceas, only that the sexual differentiation goes back to the 

 spore itself; the spores are of two kinds, large female spores which produce the 

 small female prothallium, and small male spores which produce only antherozoids. 

 The preparation for this sexual difference is manifested even in the asexual gener- 

 ation, by the sporangia producing only female or only male spores according to 

 their position. In Salvinia the preparation goes back still further, each entire cap- 

 sule producing only female or only male sporangia. It has already been pointed 

 out how in Phanerogams the embryo- sac corresponds to the large, the pollen-grain 

 to the small spore of heterosporous Vascular Cryptogams, and the endosperm to the 

 prothallium. The endosperm of Phanerogams no longer appears as an independent 

 structure, but only as a constituent part of the preceding generation ; in Angio- 

 sperms it is often from the first rudimentary and sometimes entirely absent, and the 

 female sexual cell, the embryonic vesicle, is then the immediate product of the 

 embryo-sac which corresponds to the large spore. The true sexual generation 

 therefore becomes less and less important ; as such it becomes devoid of significance, 

 while the sexual differentiation is carried back to the spore-forming generation, in 

 which it determines the formation of the sexual organs, z. e. the stamens and ovules ; 

 and, where the flowering plant is dioecious, the sexual differentiation affects the 

 entire individual, which is either male or female. In all Cryptogams, on the other 

 hand, dioecism can be displayed only in a single generation in the course of deve- 

 lopment of the individuaP. 



^ [Parthenogenesis, or the production of a fertile embryo in the female organ without any pre- 

 ceding process of impregnation, is a phenomenon of very rare occurrence in the vegetable kingdom. 

 The best-known instance among Phanerogams is that of Ccelebogyne ilicifolia (see Smith in Trans. 

 Linn. Soc. vol.XVlII, p. 510), on which however some doubt still rests. Among Cryptogams, Chara 

 crinita is stated to produce spores capable of germinating from female plants without any access of 

 the male element. The best authenticated cases are recorded by Pringsheim mSap-oleguiaftrax and 



