A FOG A MY AND APOSPORV 57 



well ask whether such an interpretation does not read into the facts more 

 than actually exists ? If Isoetes were a plant which habitually showed 

 combined apospory and apogamy, and if various steps were present leading 

 towards the extreme result, then the conclusions might be accepted. But 

 Isoetes is a plant which is structurally stable as a rule, and there is in 

 these abnormal growths no prothalloid tissue at all. Thus they appear to 

 be merely sporophytic buds formed from sporophyte tissue, and having 

 sporophytic character throughout. They will rank with those sporophytic 

 buds which are found arising from the sorus in various Ferns, or from 

 the nucellus in some Phanerogams : they are, in fact, a mode of vegetative 

 continuance of the neutral generation, and nothing more. 



The question necessarily presents itself, what is the cytological state 

 of the tissues in the plants which show those vegetative transitions from 

 one generation to another, such as have been described for the Mosses 

 and Ferns above named? The facts would appear to be inconsistent with 

 the structural distinction of the two generations, since the acts of sexuality 

 and of spore-formation, by which the cytological changes are normally 

 effected, are liable to be omitted. It will be important to know how 

 far the distinction between the haploid and the diploid phases will remain 

 valid. The facts have lately been elucidated for a number of the 

 abnormal Ferns by Prof. Farmer and Miss Digby, 1 and for the very 

 peculiar case of the genus Marsilia by Prof. Strasburger.- 



Taking first the case of apogamy: already in 1898 Dr. Lang had observed 

 in prothalli of Scolopendrium, in the tissues bordering on the change 

 from gametophyte to sporophyte, the frequent presence of two nuclei in a 

 single cell (Fig. 36). More detailed observations have since been made on 

 other apogamous Ferns, by examination of very young prothalli, before any 

 apogamous growths had begun to manifest themselves. 3 Similar cells with 

 two nuclei were observed in the case of prothalli of Lastraea pseudo-mas, 

 var. polydactyla ; but it was shown that when two nuclei are seen in a single 

 cell a neighbouring cell is without one, and cases were found where the 

 passage of the nucleus through the cell-wall was actually in progress 

 (Fig. 39). This process is regarded as a kind of irregular fertilisation, 

 for ultimately the two nuclei fuse. On their division the nuclei of the 

 apogamous growth thus produced show, as a consequence of the fusion, 

 evidence consistent with a doubling of the chromosomes, just as it 

 happens in the normal post-sexual stage. But instead of one cell only 

 serving as the starting-point for the new generation, a number of such 

 units co-operate loosely to produce it. These results have their interesting 

 bearing on the irregularity of number, and the sporadic position of the 

 parts in such cases as those observed by Lang. It is thus seen that e\vn 

 in these irregular examples the cytological criterion between the two 

 generations may hold, and the structural limit will be found in the cells 



\Aitn. of Boi., xxi., p. 161. - /-'lora, 1907, p. 123. 



"Farmer, Moore, and Digby, Roy. Sot, Proc., Ixxi., 1903, p. 45 }. 



