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 distinction ; 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. 



