Fecundation in Orchidea 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 economj'^ 

 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 Orchideae, 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 1 shall have 

 to adduce on the subject of Asclepiadeae, makes me hesitate 

 still more to adopt their statements. 



I may also remark that in Orchideae 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 Coniferae, in which the outer coat regularly bursts 

 and is deciduous ; and it will hereafter appear, that the structure 

 in Asclepiadeae confirms the correctness of this view. 



* Annal. des Sc. Nat. xxi. p. 329. f See Additional Observations. 



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