ORGANS OF REPRODUCTION. Ill 



The summit is well contrived for retaining the pol- 

 len that may fall upon it, from being without any 

 rind to cover it and in all cases moistened with a 

 clammy fluid, which causes the grains of pollen to 

 swell, burst, and discharge their minute granules. 

 Some suppose that these are taken up by spongelets 

 in the summit similar to those of the root, while others 

 allege that the fluid matter in which the granules float 

 is sucked up. 



Professor Amici discovered in 1823, that the grains 

 of pollen, when shed on the summit, do not burst as 

 they do in water, but in a few hours shoot out one or 

 more very delicate tubes, which penetrate the tissue of 

 the summit, and extend themselves down through the 

 interior of the style, as far as the seed organ, where 

 they expand between and around the nascent seeds, 

 serving, it is probable, to convey thither the granules 

 which at least enter into the tubes. 



Mr. R. Brown is of opinion that the tubes, or the 

 granules which they convey, only produce a stimu- 

 lating effect on the nascent seeds, without penetrating 

 their texture. In the swallow-worts the grains of pol- 

 len are inclosed in a kind of sac, the most prominent 

 part of the convex edge of which is applied to the 

 summit of the pistil as fecundation is about to take 

 place, when a number of extremely slender threads, 

 each being the pollen tube of a single grain, and not 

 more than the 1500th of an inch in diameter, are 

 emitted from this edge into the tissue of the pistil. 



