256 
Notes. 
growth of this modified gametophyte. This spore-producing stage 
would at first follow the sexual stage in any individual exposed to 
dry conditions. It is possible to imagine, however, how the asso- 
ciation of the asexual with the sexual individual might come about. 
Absence of fluid water would prevent the liberation of motile spores 
from the zygote. The latter would be obliged to germinate in situ , 
and the fact that it did so under dry conditions would tend to the 
shortening of the sexual stage, and the speedy assumption of the 
sporophytic form and mode of reproduction. From the spore, which 
would always separate from the parent, a sexual individual would 
arise, since germination could only take place in a damp spot. As 
soon as, with the increase in size and complexity of the spore-bearing 
plant, a vegetation capable of affording shade came into existence, 
the conditions suitable for the persistence of the more primitive, 
alga-like, sexual stage in the life-history would be present. The 
latter has, of course, also been modified in various ways. 
In the concluding portion of this paper, the theories of antithetic 
and homologous alternation are compared by considering the ex- 
planations they afford of the facts. The general conclusion reached 
is that, while both afford a possible explanation of the facts of alterna- 
tion in archegoniate plants, any evidence which would render one 
or the other untenable is wanting. The reasons on which either is 
considered more probable depend on the views held as to the lines 
of descent which have been followed, and the degree to which the 
different groups of archegoniate plants have had a common origin, 
or represent actual steps in the process of evolution of the sporo- 
phyte. Under these circumstances the question must be regarded 
as an open one until the available lines of evidence have been more 
fully investigated. 
I am especially indebted to Dr. Scott and Professor Bower for their 
assistance and advice ; the work was commenced in the Jodrell 
Laboratory of the Royal Gardens, Kew, and subsequently carried on 
in the Botanical Laboratory of the University of Glasgow. 
THE LIGULE IN LEPIDOSTROBUS.— The presence of 
a ligule on the vegetative leaves of Lepidodendron is now a well- 
ascertained fact, and it can be readily seen in several of the slides 
preserved in the Williamson Collection at the Natural History 
Museum. The best description and figures of the ligule in the 
