Mr Brown's Microscopical Observations. 339 



of the Antherae : the molecule, 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 

 Gramineae, 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 Onagrariae. 



In Asclepiadeoe, strictly so called, the mass of pollen filling each cell of 

 the anthera is in no stage separable into distinct grains; but within, its 

 tesselated or cellular membrane is filled with spherical particles, common- 

 ly 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 semi transparent, to a process of nearly similar consistence, 

 derived from the gland of the corresponding angle of the stigma. 



In Feriplocece, 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 Asclepiadese. 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 com- 

 pound, 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 glandular 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 pro- 

 perty 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 numbers, and in 

 evident motion, along with a few of the larger particles, whose motions 

 were much less manifest, and in some cases not observable *. 



* While this sheet was passing through the press, I have examined the pollen of 

 several flowers which have been immersed in weak spirit about eleven months, par- 

 ticularly of Viola tricolor, Zizania aquatica, and Zea Mays ; and in all these plants 

 the peculiar particles of the pollen, which are oval or short oblong, though some- 

 what reduced in number, retain their form perfectly, and exhibit evident motion, 

 though I think not so vivid as in those belonging to the living plant. In Viola tri- 

 color, in which, as well as in other species of the same natural section of the genus, 

 the pollen has a very remarkable form, the grain on immersion in nitric acid still 

 discharged its contents by its four angles, though with less force than in the recent 

 plant. 



