Notes. 
589 
suggested, on distinct lines from the beginning. It is therefore 
advisable to ascertain if any evidence exists which may indicate how 
the Vascular Cryptogams could have been derived directly from Algal 
forms. Something of the kind, as we shall see, may possibly be 
afforded by the facts of apogamy. 
So far we have seen no reason to regard the nature of alternation 
and the views on descent which underlie it as anything but open 
questions. There are, however, two important classes of facts, which 
have been regarded as affording more direct evidence in favour of the 
antithetic and homologous theories respectively. These are the cyto- 
logical differences between the two generations, and the deviations 
from the normal life-history known as apospory and apogamy. 
The first of these will only be mentioned. The existence of the 
double number of chromosomes, which results from the sexual fusion, 
in the nuclei of the sporophyte, throughout Bryophyta, Ferns, and the 
higher plants, certainly appears to lend support to the view that 
the sporophyte is an interpolated stage in the life-history. From 
the cytological point of view the intercalation is between the 
doubling of the number of chromosomes by the sexual fusion and 
the reduction in number in the spore mother-cells. Facts are 
wanting as to the nuclear changes in Thallophytes, and also in 
apogamy and apospory. 
These latter phenomena are the last element in the problem that 
can be referred to at length. We saw that in the case of the 
alternation of clearly homologous generations in the Thallophyta 
it had been shown that the assumption of the sexual or asexual 
form depends on the external conditions. This experimental study 
needs to be extended to the rudimentary sporophytes of the Green 
Algae, but with regard to these it is already known that in Oedogonium 
the fertilized ovum may grow out directly into a vegetative plant, 
instead of dividing into spores. In the Archegoniatae this complete 
substitution of one generation for another is not known to occur ; 
no variations in the external conditions are known to induce a Fern- 
spore to develop into a Fern-plant, or the fertilized ovum to give rise 
directly to a prothallus. But the facts as to the direct development 
of the one generation from the tissues of the other, and the existence 
of structures which may fairly be described as intermediate between 
gametophyte and sporophyte are sufficiently striking. 
The main facts with regard to apospory, the vegetative origin of the 
