4 Lawson . — The Gametophytes , Fertilization and 
nuclei. A similar condition has been reported for Taxns (Coker, ’04) and 
Torreya (Robertson, ’04; Coulter and Land, ’05). Indeed, from the number 
of types that have been investigated, it would seem that the survival of the 
sterile prothallial cells among the Coniferales, is almost entirely confined to 
the Abietineae. The single exception is that of Podocarpus , and even here 
there are strong suspicions that the Podocarpeae and Abietineae are closely 
related (Coker, ’02). 
In this connexion it is interesting to note that Miss Robertson (’04) 1 
correlates the survival of the vegetative prothallial cells with the winged 
character of the pollen-grain. No one doubts the primitive character of the 
Cycads and Ginkgo , and here the prothallial cells are evident. They are 
also constantly present in the Abietineae, but in the Taxeae, Taxodineae, 
and Cupressineae they are conspicuous by their absence. Miss Robertson 
(’04) infers that the superior buoyancy produced by the bladder-like 
swellings of the exine in Podocarpus and in the Abietineae ‘ would make 
any difference in weight of very little importance, and so natural selection 
would not come into play to eliminate the vestigial sterile cells in the pollen- 
grain.’ In her explanation Miss Robertson assumes that the elimination of 
the one or two prothallial cells within the pollen-grain was accomplished 
in order to bring about a reduction in its specific gravity ; and also that it 
was easier for natural selection to develop new structures in the form of 
winged bladders than to eliminate one or two very minute cells which have 
long since become functionless. Personally, I question whether the presence 
of one or two very minute prothallial cells within the pollen-grain would 
make any perceptible difference in its specific gravity. The early reduction 
in the number and size of the cells in the prothallium was no doubt accom- 
plished to meet this end, thus affording an easy transportation ; but I am 
more inclined to believe that the final elimination of the last two function- 
less vestigial cells in the Taxodineae, Cupressineae, and Taxineae was the 
result of long disuse. My belief in this regard is strengthened by a fact 
which Miss Robertson has evidently overlooked. It is well known that in 
the Pine pollen the vestigial prothallial cells become disorganized very soon 
after they are formed, and practically vanish before the pollen is discharged 
from the sporangium. This is also true for other members of the Abietineae. 
Miyake (’03) reports that in the pollen-grain of Picea , just before pollina- 
tion, ‘ the disintegrating remains of the first two prothallial cells can be 
seen merely as two thin and darkly staining bodies between the stalk-cell 
and the pollen wall.’ With the practical disappearance of the prothallial 
cells in the winged pollen, before the bladder-like appendages have an 
opportunity of producing any buoyancy, it is difficult to understand how 
these latter structures can be responsible for the survival of the former. 
1 See also Jeffrey, E. C., The Comparative Anatomy and Phytogeny of the Coniferales, Part II. 
Mem. Bost. Soc. Nat. Hist., 1904. 
