4 MEYEN ON VEGETABLE IMPREGNATION 
We will first make ourselves acquainted with the formation 
of the young embryo, or what takes place in the true process 
of impregnation in those cases where an actual embryo-sac is 
present, and where the union of the pollen-tube with the summit 
of the embryo-sac may be more or less easily traced. 
In most cases where separate embryo-sacs occur, the formation 
of their first substratum takes place in the apex of the nucleus, 
and descends from thence towards its base or chalaza-end; in 
other cases, on the contrary, the embryo-sac is formed in the 
base of the nucleus, and then ascends upwards, as for instance 
in Viscum album; or, what far more frequently happens, the 
embryo-sac is formed out of the apex of the nucleus, and grows 
upwards to meet the entering pollen-tube. This growing to 
meet (towards) the penetrating pollen-tube of the embryo-sac, 
I regard as a phenomenon worthy of most especial attention ; 
I observed it first in Phaseolus, where the embryo-sac origi- 
nates in the apex of the nucleus, but is developed on the out- 
side of it, in the interior of the cavity of the second tunic, and 
grows with its apex directly into the endostomium*; nay, I 
even saw in this plant the remarkable phenomenon, that the 
apex of the embryo-sac grew a couple of lines in length out of 
the apertures of the tunics, because the formation of the pollen- 
tubes, and their entrance to the ovula in forced plants, had 
been retarded by continuous cold weather. I have for three 
years observed in Alsine media another phenomenon connected 
with this, to which I would likewise draw attention. In the 
first months of spring, immediately on the disappearance of 
the snow, this little plant usually flowers more or less fre- 
quently ; however, it seldom exhibits embryos before the end 
of April, as the formation of pollen-tubes is prevented by the 
low temperature, and thus the impregnation remains unaccom- 
plished. Now, at this time it is very frequently seen that 
the apex of the nucleus grows far beyond the apertures of the 
tunics, and forms itself into a large somewhat cup-shaped body, 
which becomes nearly as big as the entire circumference of the 
ovule, but subsequently falls off as far as the endostomium. In 
the month of May, on the contrary, I no longer found the above 
excrescence of the apex of the nucleus. We have likewise 
learnt from the memoir of Mr. Griffithy that in Santalum al- 
* See Meyen's Vegetable Physiology, vol. iii. pl. xv. figs. 9*, 10, 12. 
. . 5 ase . 
+ Transactions of the Linn. Soc. vol. xviii. pl. i. p. 71, &e. 
