468 THE AMERICAN NATURALIST. [Vol. XXXIX. 



smuts and of yeast cells, the fusion of nuclei in the teleutospore 

 and basidium and in the apogamous development of ferns, the 

 double fusion of polar nuclei and multiple nuclear fusions in the 

 embr)'0-sac (Corydalis) illustrate phenomena which I cannot 

 regard as sexual even though they have in them elements asso- 

 ciated with sexual processes and in certain cases may be substi- 

 tutes for a former sexual act. In none of these instances can 

 we be positive that the nuclei concerned are morphologically and 

 phylogenetically gamete nuclei. This point was discussed in 

 some detail in Section IV. It seems to me that Blackman's 

 (:04 a, p. 353) conception of the cell fusions preceding the seci- 

 dium in Phragmidium as "reduced forms of ordinary fertilization" 

 or Farmer's (: 03) explanation of apogamy in the fern "as a kind 

 of irregular fertilization" leads to a confusion of a substitute 

 process with a true sexual act. The substitute processes have 

 their true place as phenomena of apogamy. They can, however, 

 only have a sexual significance if they represent the origin of a 

 new set of gametes in the organism, a proposition which is not 

 likely to be maintained by anyone. 



3. Sporogenesis. 



We are employing the term sporogenesis, as must have been 

 apparent in preceding divisions of this paper, to designate a 

 characteristic and highly specialized type of spore formation that 

 is universal in all plants above the thallophytes. The process 

 always terminates the sporophyte phase in ontogeny of these 

 higher plants, and is especially distinguished as the period of 

 chromosome reduction in the life history. The cell activities of 

 sporogenesis are therefore of particular interest, and, since spore 

 mother-cells are generally large and their nuclear and cytoplasmic 

 structure especially clearly differentiated, we have perhaps ob- 

 tained more knowledge of mitotic phenomena from the study of 

 these elements than of any other tissues of the plant body. 



The reduction phenomena of sporogenesis have been estab- 

 lished in some forms of the thallophytes, certainly in the tetra- 

 spore mother-cell of Dictyota (Williams, :04a). There are also 

 reasons for suspecting that the oospore of CEdogonium, and the 



