466 MICROSCOPICAL OBSERVATIONS 



or more rarely, a part of the surface of the umbilical cord. 

 It also appeared, however, from some of the facts noticed in 

 the same Essay, that there were cases in which the Particles 

 contained in the grains of pollen could hardly be conveyed 

 *] to that point of the ovulum through the vessels or cel- 

 lular tissue of the ovarium ; and the knowledge of these 

 cases, as well as of the structure and economy of the antherie 

 in Asclepiadese, had led me to doubt the correctness of 

 observations made by Stiles and Gleichen upvi^ards 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 

 impregnation. 



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

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

 to enable me to pursue the investigation. Finding, how- 

 ever, in one of the few plants then examined, the figure of 

 the particles contained in the grains of pollen clearly dis- 

 cernible, and that figure n"ot spherical but oblong, I expected, 

 with some confidence, to meet with plants in other respects 

 more favorable to the inquiry, in which these particles, 

 from peculiarity of form, might be traced through their 

 whole course : and thus, perhaps, the question determined 

 whether they in any case reach the apex of the ovulum, or 

 whether their direct action is limited 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 remark- 

 ably well adapted to the object in view. 



This plant was Clarckia pulchella, of which the grains of 

 pollen, taken from antherse full grown, but before bursting, 

 were filled with particles or granules of unusually large size, 

 varying from nearly ^th to about jjigth 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 observed many of them very evidently 

 in motion ; their motion consisting not only of a chano-e of 

 place in the fluid, manifested by alterations in their relative 

 positions, but also not unfrequently of a change of form in 



