708 Mr. Brown on the Organs and Mode of 
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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 Orchidez 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 
