230 PROFESSOR ALEXANDER DICKSON ON THE EMBRYOGENY OF 
lariaceze, &c.,* probably fulfil a similar function as organs for the absorption of 
nutritive material required by the developing embryo. One cannot reflect on 
such arrangements without being struck by a certain analogy which they pre- 
sent to the provisions for foetal nutrition in the higher animals. In the feetal 
root-process of Tropwolum we have an organ of nutrition which has its parallel 
in the mammalian allantois, which forms the root-process, so to speak, by which, 
when developed into the foetal placenta, nutritive material is absorbed from the 
maternal system—the structure being in both cases a transitory portion of the 
germ itself. Again, as regards the cecal dilatations of the embryo-sac in 
Labiate, &c., we have processes which may be regarded as villi for nutritive 
absorption. Now, inasmuch as the vegetable embryo-sac is the undoubted equiva- 
lent of the animal ovwm, it is not too much to compare the processes therefrom 
with the villi of the chorion, which play so important a part in the earlier stages 
of the foetal nutrition of the higher animals. It is, of course, impossible, as it 
would be absurd, to draw more than a very general parallel between organisms 
at such a distance from each other in the series; but the comparison, such as it 
is, is very interesting and instructive. 
Not the least important fact brought out by the foregoing investigation is this, 
that in two species of Tropwolum we find normally a penetration of the carpel- 
lary tissue by the extra-seminal root-process—a phenomenon which seems to 
occur in 7. majus only as a rare exception. In the three species, 7. majus, 
T. peregrinum, and T. speciosum, we have a most striking series presented to us 
in the relations borne by the extra-seminal process to the carpel. In 7. majus 
we have the extra-seminal process normally running its whole course free in the 
cavity of the seed-vessel. In the abnormalities observed by WiLson and myself, 
however, this process insinuates itself into the carpellary substance after it has 
run the length of the ovarian cavity; in 7. peregrinum the penetration of the 
carpel is normal, but takes place at a much earlier period of development; 
while in 7. speciosum the process plunges into the tissue of the carpel imme- 
diately on making its exit from the seed. 
It would be easy here to indulge in that kind of speculation which is so 
common a feature of modern scientific thought. We might imagine that at one 
time there existed some old type, analogous, let us say, to the ordinary form of 
Tropeolum majus, where circumstances might chance to make a slight pene- 
tration of the carpel, at a comparatively late period, by the extra-seminal root, 
such as occurs in the abnormality above described, of vital importance, and 
hence capable of being made permanent by natural selection. Further, that in the 
course of time the penetration might show a tendency to take place at a some- 
what earlier period in development, such variation becoming in like manner fixed 
* Scuacut, Bot. Zeitung, 1855, pp. 646-7. In Plate XVI. fig. 26, I have given a drawing carefully 
constructed after figures by TuLasnz, showing these cecal processes in a labiate plant, Dracocephalum 
peltatum. 
