10 MEYEN ON VEGETABLE IMPREGNATION 
sac. There is no embryo-sac in the Orchidee; but the nucleus 
is converted about the time of impregnation into a thin mem- 
brane, which occupies as it were the place of the embryo-sac, 
although but for a short time, as this membrane is very soon 
re-absorbed, and the embryo is then formed within the second 
tunic. As soon as the pollen-tube in the Orchidee has pene- 
trated into the summit of the nucleus-sac, and there comes into 
contact with the contained fluid, its summit swells to a more 
globular form, and out of this originates the germ-vesicle, which 
very soon separates by constriction from the cavity of the pol- 
len-tube. This germ-vesicle expands, as in the other cases, in 
length, consequently descends deeper into the cavity of the 
nucleus, and out of the end of this much denser new cellular 
formation originates the young embryo, and not, as M. Schlei- 
den has stated, immediately out of the summit of the pollen- 
tube*. Still more similar to the former cases is the forma- 
tion of the embryo in Capsella Bursa-pastoris, in Draba verna, 
&ec., where a funiculus of considerable length always makes 
its appearance. In the former plant I was not able, from 
the firmness of the tunics at their aperture, to lay open the 
germ-vesicle at its first formation, but only after it had be- 
come converted into a small cylindrical bag, which, however, 
was four and even five times as broad as the pollen-tube still 
adhering to it, but separated from it by a septum. This new 
cylindrical bag grows longer and longer, swells more and more 
to a globular form at its end connected with the pollen-tube, 
divides below this swelling into a series of cells, and at the 
other end, which projects free into the cavity of the nucleus, is 
formed the globular cell, which is the first substratum for the 
embryo. A long series of drawings, which represent the act of 
impregnation, and the formation of the embryo in Capsella 
Bursa-pastoris, 1 have given in plate xiii. of my Vegetable 
Physiology, to which work I may here refer. In Draba verna 
the funiculus is likewise very long, and the young embryo first 
originates deep in the nucleus-cavity : since the funiculus is here 
almost perfectly cylindrical, and occurs merely provided with 
some diagonal walls, it appears like an immediate continuation 
of the pollen-tube, which, however, at the time of impregnation, 
* See my illustrations on this subject in the Vegetable Physiology, vol. iii. 
pl. xiii. figs. 84, 35 and 36, and pl. xv. fig. 23. 
