SIGNIFICANCE OF THE SEXUAL PROCESS. 51 



I have quoted thus at length because it seems that this statement of 

 Strasburger is a compact and concise summing up of the phylogenetic 

 development of the process of reproduction and multiplication of indi- 

 viduals among the lower plants. 



The intercalation of new asexual reproductive organs into the origi- 

 nal generation is strikingly illustrated in many of the fungi, in which 

 the independent individualization of the different stages of development 

 of the sexual generation into special organs of vegetative multiplica- 

 tion, or even into distinct individuals, was carried so far that sexuality 

 seems to have disappeared entirely, as in the higher fungi. On the 

 other hand, in all plants beyond and including the Bryophyta there 

 arose an altogether new generation as the product of the sexual act, 

 whose function is to produce asexually a large number of individuals. 

 The degree of development attained by the new generation in the 

 plants above the Thallophyta differs according to whether its activity 

 was limited to the production of asexual spores alone, or included 

 nutritive functions as well, or whether it became an independent indi- 

 vidual. In the Bryophyta, especially in some of the simpler liver- 

 worts, the new asexual generation is confined almost exclusively to the 

 production of spores, i. e., to the multiplication of the individual, 

 while the original or sexual generation upon which all nutritive func- 

 tion is devolved, together with vegetative multiplication as well, has 

 attained in many cases a cormophytic differentiation. In the Pteri- 

 dophyta and in the higher plants, on the contrary, the center of gravity 

 of phylogenetic evolution is transferred to the new or asexual genera- 

 tion arising from the act of fecundation, and in these plants the asexual 

 generation has attained its highest cormophytic development. Among 

 the Pteridophyta of the present time it is evident that (1. c., p. 283) 

 " as this evolution took place, the nutritive apparatus of the sexual 

 generation became of less importance, and it became altogether super- 

 fluous from the moment when the asexual generation began to provide 

 its spores with the material necessary for the development of the sexual 

 generation." Along with this evolution there came into existence, as 

 a correlative phylogenetic process, the dimorphic character of the 

 gametophyte, which is characteristic of certain Pteridophyta and of 

 all Spermatophyta. This dimorphism was probably manifested in the 

 character of the mature gametophyte before any visible trace of it could 

 be recognized in the unicellular stage of the sexual generation, namely, 

 the spore. To illustrate this fact we need only to recall the condition 

 which obtains among certain homosporous Filicinece, for example, 

 Onoclea struthiopteris of the Polypodiacea. Here there is no visi- 

 ble evidence of heterospory, yet it is perfectly well known that in every 



