CHAP. VI. 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 obvio'us 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- 

 adese. 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 Asclepiadeee 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 neai-ly identi- 



