ALTERNATING GENERATIONS 



one generation and forms the starting-point for the next generation : the 

 older usage based upon this obvious fact is, therefore, to be preferred, 

 and the spore may be still held to be the obvious boundary between 

 the two generations. The gametophyte, or haploid phase, will then be 

 recognised as extending from the spore to the zygote in each cycle, and 

 it shows " n " chromosomes normally in all its nuclear divisions : the 

 sporophyte, or diploid phase, is recognised as extending from the zygote 

 to the spore, and it shows " 2n " chromosomes in all its normal nuclear 

 divisions. However difficult these nuclear details may be to recognise 

 in any given case, so far as observation goes within the limits of the 

 Archegoniatae they provide a structural basis for the distinction of the 



two generations more exact 

 than any other, a distinc- 

 tion which runs parallel 

 with those less accurate 

 criteria on which the 

 recognition of the genera- 

 tions was first founded. 



The possession of this 

 means of diagnosis neces- 

 sarily turns attention afresh 

 to those cases where the 

 transition from one genera- 

 tion to the other is bridged 

 over by direct vegetative 

 growth, viz. the phenomena 

 of Apogamy and Apospory, 

 which have figured so largely 

 in the discussions on alter- 

 nation. It has long been 



known that the two alternating generations are not always delimited from 

 one another respectively by those unicellular phases of the spore and the 

 zygote; but that in certain cases, and among the Archegoniatae most 

 commonly in the Ferns, there may be a vegetative transition either from 

 the prothallus to the sporophyte without the intervention of a sexual 

 process this is termed apogamy; or conversely, from the sporophyte to the 

 prothallus without the intervention of spores this is designated apospory. 



In the most frequent examples of apogamy in Ferns the place of an 

 embryo is taken by a process which originates from the tissue of the 

 cushion as a result of vegetative growth and division of its cells (Fig. 33, A) : 

 it soon takes a form corresponding to that of an embryo, with first leaf, 

 root, and apex of axis (Fig. 33, B and c), and it finally becomes an established 

 plant in the same way as those sexually produced (Fig. 33, D). In some 

 cases these developments may take place in entire absence of archegonia 

 on the prothallus; in others various conditions of the archegonia may be 



FIG. 34. 



Scolopendrium vulgare. Prothallus from the branched cylindrical 

 process of which ten roots arose : eight of these are visible in the 

 drawing. X about 6. ('After Lang.) 



