The Taxoidece. 
97 
Ginkgo , and the Abietineae, while it is absent or very poorly 
developed in the Taxoideae. While admitting the possibility that 
the megaspore membrane may have the phylogenetic value which 
Dr. Thomson attributes to it, I should like to point out that it is by no 
means certain that it is not an adaptive character. The Cycads have 
a very large and watery prothallus and they are characteristically 
xerophytes, so that in their case the well-developed megaspore 
membrane may have an important protective function. Also the 
fact that according to Dr. Thomson’s observations the megaspore 
membrane is always associated with a tapetum, suggests that it 
may have some significance in connection with nutrition. 
There are two other points in the structure of the female 
gametophyte of Taxoidece on which Professor A. A. Lawson has 
laid some stress in his study of Cephalotaxus (26), as showing that 
this genus “ represents a very recent type of Conifer.” The first 
of these is the absence of the ventral canal cell. In Cephalotaxus 
there is a ventral canal nucleus, but it is not separated by a wall 
from the egg cell, while in Taxus and Tovveya even the nucleus 
occurs rarely or never. In the Abietineas on the other hand a well- 
marked ventral canal cell is cut off. The second point is that 
“ prothallial” nuclei are absent from the pollen grains of Taxoideae, 
while they are present in the Abietineae. Lawson concludes “ If 
the retention of such evanescent vestigial structures as the 
prothallial cells in the pollen or the membrane of the ventral canal¬ 
cell has any phylogenetic bearing, then we have an argument in 
support of the primitive character of the Abietineae as a group.” 
Another point which might perhaps be brought forward is that the 
differentiation between the two sperm nuclei in Taxus and Torreya 
is a decidedly sophisticated character. All these arguments seem to 
me to carry considerable weight, and to show at least that the 
Taxoideae have specialised a great deal along their own particular lines, 
but nevertheless I am disposed to think that the balance of evidence 
is in favour of the view that the Taxoideae have retained a greater 
number of primitive characters than the Abietineae. One point 
which Professor Lawson brings forward, the absence of prothallial 
cells in the pollen grains of Taxoideae, may, as I have tried to show 
elsewhere, (Robertson, 16, p. 139) find its explanation in the almost 
universal association of the winged character of the pollen grain 
with the presence of prothallial nuclei. (For criticisms of this 
hypothesis see Seward and Ford, 24, p. 392 and Lawson, 26, p. 4.) 
The problem of the relation of the three genera of Taxoideae to one 
