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, 
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 
