52 INTRODUCTION. 



culture of spores some will develop into distinctively male prothallia, 

 bearing only antheridia, while others show a marked tendency to 

 develop into prothallia bearing only archegonia. It is also well 

 known that this tendency toward dimorphism is, in a measure, influ- 

 enced by external conditions, for if spores of Onoclea strtithiopteris 

 be sown thickly, and the culture be poorly illuminated and, conse- 

 quently, poorly nourished, the vast majority of the prothallia will be 

 male ; but if the spores be sown thinly and well illuminated, a much 

 greater number will become female plants. 



In all existing forms in which the spores, or unicellular condition 

 of the sexual generation, contain food material for the development of 

 the asexual generation, or its earlier stages, dimorphism is well estab- 

 lished, z. <?., those forms are heterosporous, and the conclusion which 

 most naturally follows is that heterospory and the disappearance of 

 the nutritive apparatus of the sexual generation represent correlative 

 phylogenetic processes. 



Now, during this phylogenetic evolution and, as Strasburger very 

 clearly puts it, 



In accordance with the general law which determines the phylogenetic disap- 

 pearance of organs which have become useless, the vegetative parts of the sexual 

 generation became more and more reduced, until little was left but the repro- 

 ductive organs themselves : hence the progressive reduction in the prothallium 

 from the Ferns up to the Phanerogams. This reduction culminated in the 

 complete loss of independent existence by the sexual generation, because it had 

 ceased to be able to nourish itself independently, and [because of] its becoming 

 enclosed by the asexual generation. In consequence of this enclosure of the 

 sexual in the asexual generation, the advantageous rapid multiplication of indi- 

 viduals which the latter originally effected was lost : in order to compensate for 

 this loss, a large number of seeds were produced in the Phanerogams in place 

 of the numerous spores of the Cryptogams ; that is, multiplication is effected 

 now by the product of fertilization instead of by asexual spores. 



In harmony with this doctrine, an alternation of generations is neces- 

 sary in those plants in which the fecundated egg gives rise to the 

 asexual generation, and the asexual spore to the sexual generation. 



The development of the plant kingdom, at least so far as sexuality 

 is concerned, seems to show that sexual differentiation was preceded 

 by asexuality, and in those groups in which a true alternation of gen- 

 erations exists the sexual generation is to be regarded as the older 

 and more primitive and as having arisen from an asexual form. In 

 fact, we are able to trace this phylogenetic development step by step, 

 or the evidence at hand, at least, seems to be sufficiently conclusive to 

 justify the general acceptance of the doctrine. Probably the first indi- 

 cation of this development is to be found among such algae as CEdo- 



