FUNCTION.] POLLEN-TUBES AND FORAMEN. 223 



the pollen to be put in communication with the foramen of 

 the ovule, through the intervention of the conducting tissue 

 of the style. In ordinary cases this is easily effected, in 

 consequence of the foramen being actually in contact with 

 the placenta. Where it is otherwise, nature has provided 

 some curious contrivances for bringing about the necessary 

 contact. In Euphorbia Lathyris the apex of the nucleus is 

 protruded far beyond the foramen, so as to lie within a kind 

 of hood-like expansion of the placenta : in all campylotropal 

 ovules the foramen is bent downwards, by the unequal growth 

 of the two sides, so as to come in contact with the conducting 

 tissue j and in Statice Armeria, Daphne Laureola, and some 

 other plants, the surface of the conducting tissue actually 

 elongates and stops up the mouth of the ovule, while fertilisa- 

 tion is taking effect. A different arrangement occurs in 

 Helianthemum. In plants of that genus the foramen is at 

 the end of the ovule 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 supposed 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 in the base of the style, 

 which hang down in the cavity of the ovary, and, floating 

 there, convey the influence of the pollen to the points of the 

 nuclei. So, again, in Asclepiads. 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 Asclepiads 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 fertilisation are lengthened 

 and emitted ; but he did not discover that these tails are only 

 formed subsequently to the commencement of a new vital 



