IN OHCHIDEjE and ASCLEPIADEiE. 509 



ovulum. M. Du Petit Thouars also, in his account already 

 quoted of these cords, supposed by him to belong to the 

 stigma of Orchidese, describes their ultimate 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 Asclepiadese, makes 

 me hesitate still more to adopt their statements. 



I may also remark that in Orchidese the six cords are 

 to be met with even in the i-ipe capsule, in which, allow- 

 ance being made for the effect of pressure, they are not 

 materially reduced in size ; and the statement by M. Du 

 Petit Thouars, of the lateral branches separating the ovula 

 into irregular groups, is certainly 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. 

 Brongniart'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 oc- 

 casionally 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 Ascle- 

 piadese confirms the correctness of this view. 



But Avhatever opinion may be entertained as to the tros 

 origin of the tube, it can hardly be questioned that its pro- 

 duction or growth is a vital action excited in the grain by 

 the application of an external stimulus. The appropriate 

 and most powerful stimulus to this action is no doubt con- 

 tact, at the proper period, with the secretion or surface of 

 the stigma of the same species. Many facts, however, and 

 among others the existence of hybrid plants, prove that 

 this is not the only stimulus capable of producing the 

 effect ; and in Orchidese I have found that the action in 



' See Additional Observations. 



