Notes. 
5 8 4 
purposes of the discussion. Instead of advocating either the theory 
of antithetic or of homologous alternation, I shall try to present 
a dissection of the subject ; with this object the main facts known 
as to alternation of generations will be briefly discussed, and the 
possible interpretations of them considered. The facts will as far 
as possible be kept apart from the theoretical views to which they 
have given rise, and the points on which our knowledge is deficient 
will be emphasized rather than minimized. 
The general facts regarding alternation of generations in arche- 
goniate plants can be dismissed very briefly. In all the main groups 
a definite alternation of a sexual with an asexual generation is found. 
The latter is normally developed from the fertilized ovum, the former 
from the spore. The Bryophyta and Pteridophyta are, however, 
opposed to one another in the relative complexity attained by the 
two generations. The sporophyte in the Bryophyta remains dependent 
on the Moss or Liverwort plant, and has as its main function the 
production of the spores. It may, however, attain very considerable 
complexity of structure and possess a well developed assimilation 
tissue. In both Hepaticae and Muscineae very simple sporogonia 
lead on to complex ones in which the sterile tissue of the wall, foot, 
seta, &c., forms a considerable proportion of the whole structure. 
The ' gametophyte, on the other hand, is always independent and 
often shows a complicated external form with clearly differentiated 
stem and leaves. In the Vascular Cryptogams also the gametophyte 
is always independent, but is of relatively simple form and structure. 
The sporophyte, which develops from the fertilized ovum, very soon 
produces roots, and attains independence by the death of the pro- 
thallus. It shows a distinction of stem and leaf, is highly organized, 
and does not develop spores until after a period of vegetative growth. 
While these points of difference which indicate the great gap between 
the Bryophyta and Pteridophyta are borne in mind, due weight must 
be given to the points of agreement.' Of these, the similar structure 
of the sexual organs, the fact that in both the sporophyte is at first 
dependent on the gametophyte, the presence of stomata and inter- 
cellular spaces in the sporophyte, and the similarity in the spore- 
production may be mentioned. A consideration of these facts by 
themselves indicates no view as to the mode of origin of the two 
generations. At no stage do the two generations in any Archegoniate 
closely resemble one another, except in the case of the young plant 
