Mr. Hassall on the Structure of the Pollen Granule. 99 



vacete, in a species of Hibiscus, H. africanus. He represents 

 them as existing in most hispid pollen, the apertures in which 

 for the passage of the pollen tubes they close, being placed 

 between the extine and intine, with their convex surfaces rest- 

 ing upon the latter. If the pollen of the above-named species of 

 Hibiscus be allowed to remain in water for a short time, the 

 pollen tubes will be seen to have emerged some distance from 

 the intine ; and upon the extremities of many of them those 

 pieces of membrane will be noticed, which are however more 

 frequently concavo-convex than plano-convex. Although I 

 have not seen these bodies closing the apertures in the extine, 

 it may be inferred with tolerable certainty that they perform 

 the office ascribed to them, from their position at the termi- 

 nations of the pollen tubes. Those who may wish to observe 

 them will not experience the same difficulty that I have 

 done, knowing the species and the manner in which to look 

 for them. They are not present in the pollen of the Ipo- 

 mceae. 



The surface of all pollen is covered with a thick tenacious 

 matter, which, according to Mohl, is most abundant upon that 

 which is hispid ; but it is at least as much so on the pollen 

 of many species of Liliacece, which are not hispid, but reti- 

 culated. If this viscid substance is to be regarded as a se- 

 cretion, then must the extine be provided with glands, or 

 some other peculiar organization for its formation ; for a true 

 secretion can only result from organization of some kind or 

 other, and, in the vegetable kingdom, may be defined to be 

 a new product, eliminated from the sap through the instru- 

 mentality of glands or other structural media. My own 

 opinion is, that it is a secretion, if not formed by the external 

 membrane of the pollen grain itself, derived from the cell in 

 which it is originally developed. I was at first inclined to 

 consider it as a mere exudation, consisting of the thinner 

 parts of the fovilla ; but its appearance and consistence differ 

 so much from this, that I believe the opinion to be scarcely 

 tenable. This secretion fulfills an important indication in 

 preventing the too easy dispersion of the pollen granules, 

 which it slightly holds together, and which would be scatter- 

 ed far and near, wide of its destined mark, by every breath 

 of wind. 



Pollen granules, though usually separate, are occasionally 

 found united. This union is either temporary, the medium 

 which retains them in contact with each other being either a 

 tenacious secretion, or filaments of the cellular tissue derived 

 from the breaking up of the cells in which the granules are 

 originally developed ; or it is permanent and organic. We 



H 2 



