“ Predisposition ” and “ Immunity * in Plants. 327 
know that the latter contain enzymes 1 , it would have to be inferred, 
because some pollen-tubes (e.g. Alopecurus) pierce and travel in the 
middle lamella, and others (e.g. Agrostemma) penetrate the cell- 
walls of the stigmatic cells, and pass through from cell to cell of 
the style 2 in a manner exactly analogous to the passage of a 
Botrytis hypha in the substance of the cell-wall 3 . Moreover, no 
one who has traced the corrosive action of the pollen-tube of Pinus 
in the tissues of the nucellus 4 , or that of a pollen-tube of a chala- 
zogamic plant in the nucellus of the ovule (e.g. Casuarina 5 ) can 
hesitate to compare the action to that of a parasitic fungus. 
Even more striking in some respects is the behaviour of the 
pollen-grains of Vaccinium and the spores of Sclerotinia, a fungus 
parasitic on the Vaccinium. As Woronin showed 6 , insects convey 
both the fungus-spores and the legitimate pollen to the stigma 
of the Vaccinium. Here they both germinate, side by side, and 
the germ-tubes of the fungus race the pollen-tubes down the style 
to the ovules. 
Now we may clearly compare this case to that of two different 
pollens placed side by side on the same stigma, and it will be 
remembered that Charles Darwin 7 proved that several orders of 
events may follow according to the origin of the two pollens 
employed in such an experiment. 
The case which chiefly concerns us here is that in which the 
prepotent pollen is that which has originated from a plant of the 
same species or variety as the one to be pollinated. Darwin cited 
several cases where it was found that if the pollen from a given 
stamen was placed on the stigma of its own flower, together with 
pollen from another flower of the same species or variety, the 
former pollen was prepotent. This case affects the present question 
less directly than the following, however, because it turns on the 
origin of the pollen from the individual plant or flower. 
Many cases are known where the stigma of a given plant A 
may be successfully pollinated by pollen from its own flower, or 
from a flower on another plant of the same species or variety, but 
will be either only partially or not at all successfully pollinated by 
pollen from a flower of another variety or species B of the same 
genus. 
It is a sort of rule that the best results in cross-breeding are 
1 Green, Phil. Trans. 185, 1894 B. p. 385. 
2 Strasburger, Befruchtungsvorgang bei den Plianerogamen, 1884, Taf. 1, Figs. 
55, 56. 
3 Marshall Ward, Ann. Bot. 1888, Yol. ii., Plate xxiv. 
4 Strasburger, Angiosp. u. d. Gymnosp. 1879. 
5 Treub, Ann. du Jard. Bot. de Buitenzorg. x. p. 145; Benson, Linn. Trans. 
1893, p. 409. 
6 Woronin, Mem. de VAcad. Imp. de St Pet. T. xxxvi. 1888, p. 25. 
7 Darwin, Cross and Self-fertilization in Plants, 1876. 
