50 INTRODUCTION. 



The simplest and most primitive organisms known reproduce them- 

 selves asexually, and we are obliged to assume that, from a phylo- 

 genetic standpoint, sexually differentiated organisms were descended 

 from asexual forms. The process of this descent is clearly illustrated 

 by certain of the green algae in which the sexual act consists in the 

 fusion of exactly similar motile gametes. These gametes were un- 

 doubtedly derived from asexual swarm spores, which they closely 

 resemble, except in that they are smaller and often have fewer cilia. 

 In Ulothrix, for example, and in many of the green algae, the gametes 

 are, so far as is known, smaller and possess only two cilia, while the 

 larger asexual swarm spores bear four cilia. Both sporangia and 

 gametangia are homologous structures, and, so far as is known, the 

 gametes differ only physiologically from the asexual spores. 



According to Strasburger, to use the language of the translation r 1 



The sexually differentiated plants manifest certain differences in their onto- 

 geny, from which it is possible to infer what was the course along which the 

 phylogenetic differentiation proceeded after sexual differentiation had taken 

 place. The simplest case is that in which the product of fertilization gives rise 

 to an individual similar to those which gave rise to the product of fertilization, 

 and which closes its own life history with the development either of sexual 

 organs or of asexual organs homologous with them. This occurs in many 

 ChlorophycecE, where, from the zygospore (the product of the coalescence of 

 similar gametes) or the oospore (the product of the coalescence of dissimilar 

 spermatozoids and ova), a generation is developed which resembles the preced- 

 ing and gives rise either to swarm-spores or to sexual cells homologous with 

 them. Generally, any one sexual generation follows after a number of asexual 

 generations, the relation being, however, dependent on external conditions, so 

 that, as Klebs has shown, the development of a sexual or an asexual generation 

 can be determined by the observer. In such cases there is a homogeneous 

 sequence of generations which does not include any other kind of sequence or 

 alternation beyond the development either of asexual reproductive organs or 

 of sexual organs homologous with them. The asexual reproductive organs are 

 especially concerned with the rapid multiplication of individuals under favorable 

 external conditions ; whilst sexual reproduction is of importance in maintaining 

 the existence of the species under circumstances which are unfavorable to the 

 vegetative existence of the individual. At the same time, sexual reproduction 

 ensures certain advantages arising from the coalescence of distinct sexual cells. 

 In proportion as the asexual mode of reproduction was replaced by the 

 sexual, the numerical conditions of multiplication were maintained either by 

 the development of a number of oospores, as in certain Fucaceae ; or, in addi- 

 tion to the sexual organs, altogether new organs were developed to ensure rapid 

 and vigorous development of new individuals in an asexual manner. This 

 took place in various ways. Either asexual reproductive organs were inter- 

 calated in the life history of the original generation, or an altogether new 

 asexual generation was developed from the product of the sexual act. 



1 English translation, Ann. Bot., 8 : 281-316. 



