the Structure of Pollen. 401 



pollen-grains possessing a smooth surface. It was supposed 

 by Guillemin to be a slit in the outer tunic intended to faci- 

 litate the admission of water into the interior of the grains, 

 and the emission of their fovilla. The observations which I 

 have made in reference to this point have shown that the lon- 

 gitudinal line has not, in any case, the appearance of a slit, 

 as it has greater opacity, when the grain is dry, than any other 

 part ; and were it a slit, it would not disappear under the ac- 

 tion of water, (which is the case,) but, on the contrary, as the 

 pollen-grain enlarged, it would become more patulous. It 

 would therefore appear that the true nature of this linear 

 marking is, that in the diy state the outer membrane is de- 

 pressed and folded in, so as to form a furrow ; but that when 

 moisture is applied the grain swells, the fold is expanded, and 

 finally disappears. This statement is confirmed by the ap- 

 pearances which I found to be presented by the pollen of 

 Antirrhinum majus. In the dry state its form is cylindrical, 

 but under the action of water it swells and becomes spherical ; 

 the furrow, at the same time, is seen gradually to unfold and 

 at last to disappear (fig. 9, 10, 11, 12). Moreover, in the 

 early stages of its development, when surrounded with fluid, 

 and being therefore in a moist condition, no furrow is per- 

 ceptible. 



Besides the several proximate principles which have been 

 shown by chemists to exist in pollen, I have succeeded in de- 

 tecting in certain species a very marked quantity of carbo- 

 nate of potassa. Whilst making an observation on the pollen 

 of Antirrhinum majus immersed in dilute sulphuric acid, the 

 field of the microscope became crowded with a quantity of 

 acicular crystals. As the fluid gradually evaporated, the pol- 

 len-grains were found to act as nuclei, fi'om which the cry- 

 stals radiated in all directions (fig. 13.). Their form appeared 

 to be that of quadrilateral prisms with obliquely truncated 

 extremities. The portion of this salt was of course infinitely 

 too small to afford a sufficient quantity for analysis ; the only 

 method therefore of determining its constitution was to dis- 

 cover the form of its crystal, and that of other salts which 

 might be formed by its decomposition. It was obvious that 

 it must be a sulphate of some base; and as potassa is an 



Ann. Nat. Hist. Vol. 2. No. 12. Feb. 1839. 2 e 



