52 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 
Fic. 34. of Apogamy and Apospory, 
Scolopendrium vulgare. Prothallus from the branched cylindrical Which have figured so largely 
1 Sen eee Acie of these are visible in the i, 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 afogamy ; or conversely, from the sporophyte to the 
prothallus without the intervention of spores—this is designated afospory. 
In the most frequent examples of afogamy 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 
