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



find the first mode of union in the pollen grains of the Epi- 

 tobia^^, Salpiglossis atrojmrjmrea, and in LechenauJtia formosa, 

 while the second is met with in all the genera of the natural 

 order EricacecB which I have had an opportunity of exami- 

 ning, in the Acacice, Mimosa, Junci, many species of Orchi- 

 dacecB, and in Oxyanthvs speciosa. Some of these grains^ 

 though they usually are permanently attached, are yet ca- 

 pable of separation; but others of them again, as the Eri- 

 cacece, &c., are so intimately joined, that they never become 

 separated. The number of pollen grains thus united together 

 is definite, being either four, or multiples of four up to sixteen, 

 but the first number is of the most frequent occurrence; thus 

 there are four in the Ericacece, Epilobia, Junci, OrchidcLcecSy 

 Oxyanthus speciosa, and Saljnglossis atropjurpurea, Lechenaul- 

 tia formosa and Mimosa mexicana. In Acacia rigens there 

 are eight, disposed on the same plane ; in Acacia decipiens 

 twelve, six being disposed on the same plane, while three are 

 placed in the centre, on either side of the flat figure so formed ; 

 and in Acacia linearis there are sixteen, eight upon the same 

 plane and four on either side. It is to be observed, that dark 

 lines run between every four grains, indicating either their 

 original separation or their tendency to become so separated. 

 When the union of pollen grains occurs in fours, they are 

 either disposed upon the same plane, as in the Epilobia and Le- 

 chenaultia formosa, or they appear as if one was placed upon 

 the other three, all the granules bearing precisely the same 

 relations to each other, and whichever one is uppermost, the 

 rest being similarly circumstanced in reference to it. One ex- 

 ception occurs to the law of the union of pollen grains in fours 

 in Epilobinm roseum, where they generally unite in threes. 



As a rule, but one mode of arrangement prevails among 

 the pollen grains of the same species. 



Pollen grains are often held together by a thready sub- 

 stance, supposed to be derived from the rupture and breaking 

 up of the cells in which they are primarily formed, in the 

 meshes of which they become either entangled, or to M'hich 

 they are attached by the secretion which covers their surface. 

 It appears to me that neither the origin nor use of these 

 threads are rightly made out. They are found in great abun- 

 dance in the pollen of the Ericacece ; Fuchsia, (Enothera, and 

 other Onagraria. 



The size of the pollen granule differs as materially as does 

 its form and structure, as will be obvious from a perusal of 

 the following table of relative sizes, although in the same 



* The granules in Epilobinm nngustifolium arc not united, resembling 

 much more those of the Clarkias than the Epilobiunis. 



