CHAP. vr. 
FERTILISATION. 
283 
nucleus is protruded far beyond the foramen, so as to lie 
within a kind of hood-like expansion of the placenta : in all 
campulitropous ovules the foramen is bent downwards, by the 
unequal growth of the two sides, so as to come in contact with 
the conducting tissue ; and in Statice Armeria, Daphne Lau- 
reola, and some other plants, the surface of the conducting 
tissue actually elongates and stops up the mouth of the 
ovule, while fertilisation is taking effect. Another case, pre- 
senting similar apparent difficulties, occurs in Helianthemum. 
In plants of that genus the foramen is at that end of the 
ovule which is most remote from the hilum; and although 
the ovules themselves are elevated upon cords much longer 
than are usually met with, yet there is no obvious means 
provided for their coming in contact with any part through 
which the matter projected into the pollen-tubes can be sup- 
posed to descend. It has, however, been ascertained by 
Adolphe Brongniart, that, at the time when the stigma is 
covered with pollen, and fertilisation has taken effect, there is 
a bundle of threads, originating from the base of the style, 
which hang down in the cavity of the ovary, and, floating 
there, are abundantly sufficient to convey the influence of 
the pollen to the points of the nuclei. So, again, in Asclepi- 
adeae. In this tribe, from the peculiar conformation of the 
parts, and from the grains of pollen being all shut up in a 
sort of bag, out of which there seemed to be no escape, it was 
supposed that such plants must at least form an exception to 
the general rule. But before the month of November, 1828, 
the celebrated Prussian traveller and botanist, Ehrenberg, had 
discovered that the grains of pollen of Asclepiadeae acquire a 
sort of tails, which are all directed to a suture of their sac on 
the side next the stigma, and which at the period of fertilis- 
ation are lengthened and emitted ; but he did not discover 
that these tails are only formed subsequently to the commence- 
ment of a new vital action connected with fertilisation, and 
he thought that they were of a different nature from the pol- 
len-tubes of other plants: he particularly observed in Asclepias 
syriaca that the tails become exceedingly long and hang down. 
In 1831, the subject was resumed by Brown in this country, 
and by Adolphe Brongniart in France, at times so nearly identi- 
