58 
Farmer.— On Isoetes lacustris , L. 
and basipetally in Selaginella I do not regard as a fact of 
great importance. The essential point is, that in each, a 
specially reproductive portion is separated from a specially 
vegetative and nutritive portion ; the chief difference between 
the two cases lying in the greater completeness with which 
the differentiation is carried out in the case of Selaginella. It 
is natural to expect that a tissue which is required rapidly to 
give up its substance to a growing organism would not form 
a small-celled tissue. This cell-wall formation would involve 
an uneconomical expenditure, and its occurrence in the endo- 
sperm of Angiosperms may perhaps be correlated with the 
comparatively slow growth of the embryo which obtains in 
these plants. 
Evidence is not wanting to show that a localisation of the 
reproductive organs of the oophyte, such as is required to 
support the suggested explanation of the structure in the two 
cases under discussion, is exactly the rule in a great number of 
widely different forms. This is especially true of the arche- 
gonia, with which we are here more directly concerned, and 
whose restriction to the ‘cushion’ of many Fern-prothallia, to 
the ‘ saddle ’ of Salvinia , to the central part of a Pilularia- 
prothallium, and to the more specially modified archegonio- 
phores of many other plants, in each instance points in the 
same direction, and still further support is given by the Equi- 
setaceae and the Lycopodiaceae. 
It might have been expected that Gymnosperms would 
evince a more decided distinction, if these conclusions are to 
hold good, between the reproductive and vegetative portion of 
their prothallia ; but apart from the fact that our knowledge 
on the early history of this structure in these plants is not 
very detailed, it may be pointed out that apparently the re- 
duction has become so advanced, so far as the reproductive 
bodies are concerned, in the higher forms at least, that not 
more archegonial rudiments are formed than can be brought 
to maturity. Further investigation in this direction would 
probably yield interesting results. 
I think that the above facts and considerations show plainly 
