On the llxistence of Active Molecules. 359 



were cases in which the particles contained in the grains of pol- 

 len could hardly be conveyed to that point of the ovulum 

 through the vessels or cellular tissue of the ovarium ; and the 

 knowledge of these cases, as well as of the structure and eco- 

 nomy of the antherae in Asclepiadeae, had led me to doubt the 

 correctness of observations made by Stiles and Gleichen upwards 

 of sixty years ago, as well as of some very recent statements, re- 

 specting the mode of action of the pollen in the process of im- 

 pregnation. 



It was not until late in the autumn of 1826 that I could at- 

 tend to this subject ; and the season was too far advanced to 

 enable me to pursue the investigation. Finding, however, in 

 one of the few plants then examined, the figure of the particles 

 contained in the grains of pollen clearly discernible, and that 

 figure not spherical but oblong, I expected, with some confi- 

 dence, to meet with plants in other respects more favourable to 

 the inquiry, in which those particles, from peculiarity of form, 

 might be traced through their whole course : and thus, per- 

 haps, the question determined whether they in any case reach 

 the apex of the ovulum, or whether their direct action is limit- 

 ed to other parts of the female organ. 



My inquiry on this point was commenced in June 1827, and 

 the first plant examined proved in some respects remarkably 

 well adapted to the object in view. 



This plant was Clarckia pulcJiella, of which the grains of 

 pollen, taken from antherae full grown, but before bursting, were 

 filled with particles or granules of unusually large size, varying 

 from nearly 40^5 5 th to about jo^ooth of an inch in length, and 

 of a figure between cylindrical and oblong, perhaps slightly 

 flattened, and having rounded and equal extremities. While 

 examining the form of these particles immersed in water, I ob- 

 served many of them very evidently in motion ; their motion 

 consisting not only of a change of place in the fluid, manifested 

 by alterations in their relative positions, but also not unfre- 

 quently of a change of form in the particle itself; a contraction 

 or curvature taking place repeatedly about the middle of one 

 side, accompanied by a corresponding swelling or convexity on 

 the opposite side of the particle. In a few instances the pai'ticle 

 was seen to turn on its longer axis. These motions were such 



