f 
STRUCTURE AND FUNCTIONS OF THE POLLEN. 429 
from the same individual a more elaborate paper, with numerous drawings, 
which it is intended should be read before the Members of the British 
Association, meeting in Glasgow.— En. ] 
Mounr-MICHAEL, GLASNEVIN, 
Dustin, August 6th, 1840. 
My Dear Sin,— In the character of a former pupil of yours, 
I take the liberty of communicating to you some observations 
which I have made upon the structure and functions of the 
pollen, and which, I believe to be original. 
You are of course aware, that acids possess the property of 
causing the grains of pollen to dehisce. This discovery, 
mentioned with regard to the sulphuric acid, in Lindley’s 
last edition of his * Introduction,” I had previously found to 
occur with dilute nitric acid, and I have since extended the 
observation to most of the acids, as well organic as inorganic. 
The knowledge of this fact would appear naturally to lead 
to the chemical examination of the stigma, and accordingly I 
have found the stigma to be acid at some period» ‘This c curious 
fact is easily demonstrated by pressing any stigma, e 
a large fleshy one, such as that of the Turk’s cap Lily, or a 
Grevillea, between a fold of litmus paper. Your extensive 
» acquaintance with species will suggest an infinite number of 
. corroborative phenomena; thus, in Myosotis, some species of 
Symphytum, Borago, Anchusa, Polemonium, etc., you may re- 
collect the flowers being red before impregnation, and changing 
to blue afterwards ; whilein some species of Jris, the converse 
of this is found, for the petaloid stigmas, which are blue pre- 
vious to the bursting of the anthers, immediately afterwards 
become purple. In these cases, the natural colouring matter 
supplies an appropriate test for the existence of acid. 
.. I need but call to your recollection the differences which 
have existed between the observations of Brongniart and 
_ Treviranus, as respects the mode of dehiscence of the pollen ; 
the first perceiving the protrusion of a gut or tube, which he 
- imagined to descend through the intercellular passages of the 
Stigma; while the latter was never able to detect this organ. - 
Lindley reconciles this contradiction by the difference tod the 
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