TRANSACTIONS OF SECTION K, 10o3 



Professor Klebs has shown in a number of cases that the mode of reproduction is 

 largely determined by the external conditions, and can be brought under experi- 

 mental control. There is thus no doubt that these sexual and asexual individuals 

 are homologous iu the full sense of the term. But there are a number of Thallo- 

 phytes in which another stage in the life history is found, which, by its regular 

 recurrence and the position it occupies in the life cycle, susTgests a comparison with 

 the sporophyte of the simpler archegoniate plants. While in many Thallophytes 

 the fertilised ovum or the zygospore develops directly into an independent plant 

 resembling the parent, in these it first divides into a number of cells, which are 

 usually motile spores, but may form a small mass of tissue from the cells of which 

 swarm spores arise. It is sufficient to mention (Edofjonium Ci/stopus and Coleochoete 

 as organisms which show this clearly. In the life history of Sphceroplea only 

 sexual individuals and the group of swarm spores, which results from division of 

 the oospore, alternate, independent asexual individuals not being found. 



If we now consider how this second form of alternation in Thallophytes might 

 have come about, without for the moment extending our view to archegoniate 

 plants, the essential distinction of the antithetic and homologous theories will 

 Ijecome plain. Further, we shall here be dealing with a problem with regard to 

 which the work of Professor Klebs justifies the anticipation that direct evidence 

 will sooner or later be obtained. The main question at issue is, In what relation 

 does the group of spores in (Edogonium, or the small mass of tissue resulting from 

 the division of the fertilised ovum in Coleochoete, stand to the asexual individuals 

 of the same species ? There is some evidence that in this stage we see the repre- 

 sentative of an asexual individual, the vegetative body of which has become more 

 or less completely reduced. Thus occasionally in (Edogonium a vegetative indivi- 

 dual develops from the zygote ; in Uothrix the zygospore develops a rhizoid, but 

 the contents of what appears to be a rudimentary plant are wholly devoted to the 

 formation of motile spores. On this view the ceU mass in Coleochoete would be 

 regarded as a reduced thallus, all the cells of which form spores asexually. The 

 reduced generation which proceeds from the zygote would genealogically corre- 

 spond to an independent asexual individual ; and just as the latter is homologous 

 with a sexual individual, so would the four spores in OEdogonium or the cell mass 

 in Coleochoete be. This would be homologous alternation of generations. 



But the same facts can be viewed in another light. In all these cases the 

 advantage to the plant in producing almost at once a number of individuals instead 

 of one as the result of the sexual act is obvious. The division of the ovum may 

 have originated as a special adaptation to this end, and not represent a reduced 

 first neutral generation at all. In the life history of these plants there would 

 then be a stage not represented in the majority of the Thallophytes, which may in 

 this sense be spoken of as interpolated. The cell mass of Coleochoete upon tliis 

 view would not represent a less reduced neutral generation, but a more complicated 

 development of the interpolated stage, which is seen in its simplest form in (Edo- 

 gonium. This stage would not correspond to, or be homologous with, the inde- 

 pendent asexual individuals, and leaving these out of account, only one individual, 

 and the result of elaboration of its zygote would be represented in the life history. 

 The alternation would not be homologous but antithetic. 



If we now proceed to apply these two points of view to the facts of alternation 

 in the Archegoniatae the problem in its most general form is this : Is the sporo- 

 phyte in the Bryophytes and the Vascular Cryptogams to be ultimately traced 

 back to modification of a genealogical individual homologous with the gameto- 

 phyte, or is it the result of still further elaboration of an intei-polated stage more 

 or less like that seen in Coleochoete ? On the antithetic theory the sporophyte is 

 traced increasing in complexity through a series of forms illustrated by (Edogonium, 

 Coleochoete, Riccia, Marchantia, Anthoceros ; and the simplest sporophytes of the 

 Vascular Cryptogams are regarded as having been derived from a sporogonium, 

 which already possessed a considerable amount of sterile tissue. If, on the other 

 hand, we apply tlie homologous theory, several alternatives present themselves. 

 The first, which is not widely different from the antithetic theory, is that in the 

 course of its descent the sporophyte of the Vascular Cryptogams has passed through 



