150 
Birbal Sahni. 
of the same tube, however, seen in the next section of the series 
(Fig. 3), is bent sharply away from the nucellus, as if it had met an 
obstacle. This makes the penetration doubtful. The growth of 
the tube is away from the floor of the pollen-chamber. 
The pollen-chamber of one of the ovules, in addition to a couple 
of winged grains, also contains two smaller, wingless, round ones 
(Fig. 4); a third type of pollen, apparently belonging to a third 
species, and differing from these in its oval shape and smaller size, 
was seen in different ovules (not photographed). Neither of the 
two latter kinds of pollen shows any sign of germination ; each 
contains either one or two nuclei and none of them appear to 
belong properly to Ginkgo. It has not been possible to ascertain 
whether there were any fertile male Ginkgo trees in the neighbour¬ 
hood of the tree from which the ovules were collected. 
We have here an interesting case of ovules containing foreign 
pollen from as many as three distinct species, one of the grains 
being in an advanced stage of germination. Among living plants 
such an occurrence has not, so far as I am aware, been hitherto 
recorded. Of course, the chance of foreign pollen effecting an 
entrance into the pollen-chamber of Ginkgo is not so remote as 
may at first appear. As in many Conifers, so also in Ginkgo, the 
pollen-collecting mechanism is a drop of liquid, exuded from the 
micropyle and subsequently sucked in again. 1 In these circum¬ 
stances any pollen-grains or other foreign bodies which are arrested 
by the “ stigmatic drop” and are not of too great a size may find 
their way into the pollen-chamber. Especially in the case of a 
dioecious tree like Ginkgo, away from its home, and in the usual 
conditions in which it is grown in' the West, perhaps there would 
be even more chance of foreign than of normal pollen reaching the 
ovules. In fact, one may well be surprised that more instances of 
this kind have not been observed, for example, in the several genera 
of Conifers (Taxus, Cephalotaxus, Juniperus, Cupressus, Sequoia, 
etc.) possessing a similar pollen mechanism. 
As Professor Seward remarked—and herein lies the chief 
interest of this note—if a similar example were found in a fossil 
state, it would in all probability lead to a reference of the pollen- 
grains and ovule to the same species. The only record of 
foreign pollen in fossil plants known to me is that by Professor 
Oliver, 2 who in Stephanospermum akenioides figures three foreign 
1 Tison, A. “ Remarques sur les gouttelettes collectrices des ovules des 
Coniferes.” Mem. Soc. Linn. Normandie, t. 24, 1911, p. 51. 
2 Oliver, F. W., Trans. Linn. Soc. Bot. II, G, 1904, p. 376 and PI. 42, 
Fig. 15. 
