Foreign Pollen in Ginkgo and Fossil Plants. 151 
pollen-grains—found by him “in but a solitary instance”—in 
addition to a large multicellular grain which was probably the true 
pollen of Stephanos per mum. A comparison of our Figs. 3 and 4 with 
Professor Oliver’s Fig. 15 is interesting. In both cases the pollen- 
chamber contains foreign as well as normal grains, but while in the 
former the germinating pollen is known to be foreign, in Stephano- 
spermum the fact that the large grain had germinated went in support 
of the assumption that it belonged properly to that plant. Had 
the sections reproduced in Figs. 3 and 4 been from a fossil seed, in 
the absence of data pointing to the contrary the temptation to 
assign the germinating grain to the plant in whose ovule it was 
contained would have been almost irresistible. There is, however, 
no reason for throwing any doubt upon the diagnosis of the pollen 
in the fossil seed described by Professor Oliver. 
Nevertheless, the present note will serve at least to demon¬ 
strate that the mere fact of germination cannot be used in support 
of conclusions regarding the identity of fossil pollen-grains found 
enclosed in ovules. 
Although perhaps it is not surprising that a pollen-grain 
should be able to germinate in a foreign ovule, it may be of 
interest to know at what stage of its growth the (presumably) 
chemical influences of the ovule arrest the further success of the 
stranger. Is the ultimate failure occasioned by an active repulsion 
exercised by the ovule, or is it due simply to a passive inhospitality? 
Artificial pollination experiments, on a convenient plant like Taxus, 
might be of some use. 
1 wish to express my thanks to Professor Seward, at whose 
suggestion this note was written. 
The Botany School, 
Cambridge. 
May, 1915. _ 
EXPLANATION OF PHOTOGRAPHS ON PLATE II, 
Illustrating Mr. Sahni’s Paper on Foreign Pollen in the 
Ovules of Ginkqo. 
Fig. 1. Ginkgo. Median longitudinal section of young ovule. Two 
foreign pollen-grains in pollen-chamber, the left-hand one showing the two 
large wings, the other lying in a different plane. Female prothallus in early 
free nuclear stage. 
Fig. 2. Same ovule as above, the right-hand grain with its tube penetrat¬ 
ing (?) the nucellus. Starch grains. Reticulate markings on the wings. 
Fig. 3. Same ovule, next section of the series. Pollen-tube with two 
nuclei; a third nucleus is seen in Fig. 1, still enclosed in the grain. Tip of tube 
reflexed. 
Fig- 4. Another ovule. Four pollen-grains—two winged, two unwinged. 
