CHAPTER V. 



ALTERNATION IN THE THALLOPHYTES. 



THE early recognition by Hofmeister of alternation of generations as a 

 general feature in the life-cycle of the Archegoniatae naturally led Botanists 

 to enquire whether any similar succession of phases existed in other 

 plants : and the question was soon directed towards those lower in the 

 scale, which are collectively termed the Thallophytes. Notwithstanding 

 that this term covers a most heterogeneous series of organisms, a very 

 large number of them show processes of propagation analogous to those 

 seen in the Mosses and Ferns. The existence, on the one hand, of the 

 phenomenon of sex, and on the other of the means of propagation by 

 non-sexual bodies, or spores of various kinds, suggested the comparison 

 with corresponding features in the Archegoniatae. Such comparison at 

 once raises the further question how far the study of the Thallophytes 

 may throw light on the origin of those recurrent and alternating phases 

 seen in Archegoniate Plants. 



It will be well at once to realise that the phenomenon of sex, and the 

 production of germs, by which the number of individuals may be increased, 

 are not necessarily in any way connected in plants at large. It is true 

 that in certain plants, and even in large groups of them, experience shows 

 us that there is an obligatory succession of such events in the life-history r 

 liable, however, as we have seen in the Ferns, to certain exceptional 

 modifications. We know from experience that the fertilised zygote of the 

 Archegoniatae grows into the sporophyte, which has as its ultimate end 

 the production of spores : it has never yet been seen to grow directly into 

 a prothallus again. The spore of the Archegoniatae, according to our 

 invariable experience, germinates to form a prothallus : it has never 

 been seen directly to produce a new sporophyte. There is then an 

 obligatory succession of events in the life-history of the Archegoniatae. 

 External circumstances may affect the production of fertilised zygotes, or 

 of matured spores ; but so far experiment has not altered the product 

 which arises respectively from the zygote or the spore, nor has such 

 change been observed in Nature. When, however, we turn to organisms 



