510 ON THE ORGANS AND MODE OP FECUNDATION 



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 completed, render it probable that they derive nourish- 

 ment 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 untwists and resumes its original position. 



Of the changes produced in the ovulum consequent to 

 impregnation, 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 original 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 simulta- 

 neously with, the disappearance of the original nucleus, 

 and Avhile the enlargement of the whole ovulum is gradually 

 proceeding, a minute opaque round speck, generally seated 

 about the middle of the testa, becomes visible. The 

 709] opaque speck is the commencement of the future embryo. 

 At this period, or until the opaque corpuscle or nucleus has 

 acquired more than half the size it attains in the ripe seed, 

 a thread may be traced from its apex very nearly to the 

 open end of the testa, or as it may be supposed, to the 

 apex of the original nucleus of the unimpregnated ovulum. 



This thread consists of a simple series of short cells, in 

 one of which, in a single instance only however, I observed 

 a circulation of very minute granular matter ; and in seve- 

 ral cases I have been able to distinguish in these cells that 

 granular areola so frequently existing in the cells of 

 Orchideous plants, and to which I shall have occasion here- 

 after to advert. 



The lowermost joint or cell of this thread is probably the 

 original state of what afterwards, from enlargement and 



