Fecundation in Orchidea. and Asclepiadea. 733 



of fecundation ; the tubes being merely the channels conveying 

 them to the organ or surface on which they are destined to act. 



The arguments which might be adduced in favour of this, the 

 generally received, opinion, would probably be the variety in the 

 form and size of the granules in different plants, with their great 

 uniformity in these respects in the same species ; added to the 

 difficulty of conceiving in what manner the tubes themselves can 

 operate. On the other hand, their great diminution in number, 

 or even total disappearance, in Asclepiadeae and Orchideae, long 

 before the tubes have finished their growth, would afford an ar- 

 gument of some weight at least against their essential import- 

 ance in any case ; and it may be added, that in Asclepiadeae 

 there appears to be no other source of nourishment for the tube 

 until it has penetrated into the style, than these granules. Nor 

 is it necessary to suppose that the tubes themselves act directly, 

 it being even probable that they also contain a fluid or granular 

 matter much more minute than that originally filling the cavity 

 of the grain*. 



Our knowledge indeed appears to me not yet sufficient to 

 warrant even conjectures as to the form of the immediate agent 

 derived from the male organ, or the manner of its application to 

 the ovulum in the production of that series of changes consti- 

 tuting fecundation. I may however be allowed to observe, that 

 at present, with respect to this function, we are at least as far 

 advanced in these two families, hitherto considered so obscure, 

 as we are in any other tribe of Phsenogamous plants : and I 

 even venture to add, that in investigating the obscure subject 

 of generation, additional light is perhaps more likely to be de- 

 rived from a further minute and patient examination of the 

 structure and action of the sexual organs in Asclepiadeae and 

 Orchideae, than from that of any other department either of the 

 vegetable or animal kingdom. 



* See Additional Observations. 



Explanation 



