CHAPTER IV. 
CYTOLOGICAL DISTINCTION OF THE ALTERNATING 
GENERATIONS OF ARCHEGONIATAE. 
ALTERNATION is thus found to be a general phenomenon for Archegoniate 
Plants. It was at first recognised chiefly on the basis of the propagative 
organs which the alternating generations respectively bore, and the dis- 
tinction was confirmed on grounds of external form and of anatomical 
structure. The two phases, however, presented no very strict criteria by 
which they could with certainty be told apart. As regards external form, 
a foliar development was found to exist in the sexual generation of the 
Bryophytes, and again in the neutral generation of Vascular Plants: and 
however strongly it might be urged on grounds of detailed comparison 
that these were distinct in origin, and therefore only analogous, still the 
fact that foliar development exists in them both showed that external form 
did not constitute a strict criterion. As regards anatomical structure, the 
presence or absence of vascular tissue, and of intercellular spaces appeared 
at first to give a ready distinétion; but a better knowledge of the anatomy 
of the larger Mosses showed that they also contain conducting tissues closely 
analogous to the vascular strands of Pteridophytes. Again, it is a fact 
that there is an ample ventilating system in the sporophyte, and that 
intercellular spaces are generally absent in the gametophyte; but in the 
leaves of certain Filmy Ferns there may be no intercellular spaces through- 
out considerable tracts, while the statement for the gametophyte is one of 
those negative statements which are at any time open to reversal. Even the 
production of the characteristic organs of propagation, and the transition 
by spore or zygote from one generation to the other, is not so absolute a 
distinction as was once thought; for first apogamy, and later apospory 
were discovered, and it was thus seen that a vegetative transition might 
take place from either generation to the other, without the critical incident 
of production of spore or zygote intervening as a limit. The climax of 
these difficulties in definition of the two generations was reached when 
Lang described, in 1896, how in certain Ferns sporangia might Be: borne 
directly upon the prothallus itself. 
