164 Mr. R. Brown on the Existence of active Molecules 



In a great proportion of these plants I also remarked the same 

 reduction of the larger particles, and a corresponding increase 

 of the molecules after the bursting of the antherae : the mole- 

 cule, of apparently uniform size and form, being then always 

 present ; and in some cases indeed, no other particles were 

 observed, either in this or in any earlier stage of the secreting 

 organ. 



In many plants belonging to several different families, but 

 especially to Graminece^ the membrane of the grain of pollen 

 is so transparent that the motion of the larger particles within 

 the entire grain was distinctly visible ; and it was manifest also 

 at the more transparent angles, and in some cases even in the 

 body of the grain in Onagraricc. 



In Asclepiadete, strictly so called, the mass of pollen filling 

 each cell of the anthera is in no stage separable into distinct 

 grains ; but within, its tessellated or cellular membrane is filled 

 with spherical particles, commonly of two sizes. Both these 

 kinds of particles when immersed in water are generally seen in 

 vivid motion ; but the apparent motions of the larger particle 

 might in these cases perhaps be caused by the rapid oscillation 

 of the more numerous molecules. The mass of pollen in this 

 tribe of plants never bursts, but merely connects itself by a 

 determinate point, which is not unfrequently semitransparent, 

 to a process of nearly similar consistence, derived from the 

 gland of the corresponding angle of the stigma. 



In Periplocece, and in a few Apocinece, the pollen, which in 

 these plants is separable into compound grains filled with 

 spherical moving particles, is applied to processes of the stigma, 

 analogous to those of Asclepiadece. A similar economy exists 

 in Orchidece, in which the pollen-masses are always, at least 

 in the early stage, granular ; the grains, whether simple or 

 compound, containing minute, nearly spherical particles, but 

 the whole mass being, with very few exceptions, connected by 

 a determinate point of its surface with the stigma, or a glan- 

 dular process of that organ. 



Having found motion in the particles of the pollen of all the 

 living plants which I had examined, I was led next to inquire 

 whether this property continued after the death of the plant, 

 and for what length of time it was retained. 



In plants, either dried or immersed in spirit for a few 

 days only, the particles of pollen of both kinds were found 

 in motion equally evident with that observed in the living 

 plant; specimens of several plants, some of which had been 

 dried and preserved in an herbarium for upwards of twenty 

 years, and others not less than a century, still exhibited the 

 molecules or smaller spherical particles in considerable num- 

 bers, 



