Fecundation in Orchidee and Asclepiadea. 707 
actually in contact with, the ovula, are points which still remain 
undetermined. 
I am aware that Professor Amici*, who discovered in several 
plants the remarkable fact of the penetration of the pollen tubes 
into the cavity of the ovarium, and who regards this economy 
as being very general, likewise believes that in all cases a pollen 
tube comes in contact with an ovulum. M. Du Petit Thouars 
also, in his account already quoted of these cords, supposed by 
him to belong to the stigma of Orchide:, describes their ulti- 
mate ramifications as mixing with the ovula. 
I do not however consider myself so far advanced as these 
observers in this very important point* ; and what I shall have 
to adduce on the subject of Asclepiadez, makes me hesitate 
still more to adopt their statements. 
I may also remark that in Orchidez the six cords are to be 
met with even in the ripe capsule, in which, allowance being 
made for the effect of pressure, they are not materially reduced 
in size; and the statement by M. Du Petit Thouars, of the late- 
ral branches separating the ovula into irregular groups, is cer- 
tainly not altogether correct; these groups being equally distinct 
before the existence of the cords. 
With regard to the question of the origin of the pollen tubes, 
several arguments might be adduced in favour of M. Brongni- 
art's opinion ; which is, that they belong to the inner membrane 
of the grain, the intimate cohesion of the two membranes being 
assumed in most cases, and the no less intimate union of the 
constituent parts of compound grains in some others. That an 
inner membrane does occasionally exist is manifest in the pollen 
of several Conifere, in which the outer coat regularly bursts 
and is deciduous; and it will hereafter appear, that the structure 
in Asclepiadez confirms the correctness of this view. 
* Annal. des Sc. Nat. xxi. p. 399. 1 See Additional Observations. B 
: ut 
