708 Mr. Brown on the Organs and Mode of 



But whatever opinion may be entertained as to the origin of 

 the tube, it can hardly be questioned that its production 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 contact, 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 pro- 

 ducing the effect ; and in Orchideae I have found that the action 

 in the pollen of one species may be excited by the stigma of 

 another belonging to a very different tribe. 



The elongation of the tubes, so remarkable in this family, and 

 their separation from the grain long before their growth is com- 

 pleted, render it probable that they derive nourishment either 

 from the particles contained in the grain, or from the conducting 

 surfaces with which they are in contact. 



The first visible effect of the action of the pollen on the 

 stigma is the enlargement of the ovarium, which, in cases where 

 it was reversed by torsion in the flowering state, generally un- 

 twists and resumes its original position. 



Of the changes produced in the ovulum consequent to im- 

 pregnation, the first consists in its enlargement merely ; and in 

 the few cases where the nucleus is at this period still partially 

 exposed, it becomes completely covered by the testa, the ori- 

 ginal apex, but now the lower extremity of which continues 

 open. The next change consists in the disappearance of the 

 nucleus, probably from its acquiring greater transparency, and 

 becoming confluent with the substance of the testa. Soon after, 

 or perhaps simultaneously with, the disappearance of the ori- 

 ginal nucleus, and while the enlargement of the whole ovulum 

 is gradually proceeding, a minute opake round speck, generally 

 seated about the middle of the testa, becomes visible. The 



opake 



