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 Oxoclea struthiopterds 
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. e., 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 phgleuciene 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 i 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 alge as Gdo- 
