94 Mr. H assail on the Structure of the Pollen Granule. 



The structure, form, and size of the pollen granule varies 

 considerably in different tribes of plants. 



There are, however, certain types of structure and of form 

 which occur more frequently than others. Of these, the cy- 

 lindrical and elliptical are by far the most common ; and next, 

 in frequency of occurrence, are those granules of either a tri- 

 angular or circular form, which are met with in nearly an equal 

 proportion. A great variety of other forms are to be noticed, 

 which it is the object of this paper especially to describe. 



The term cylindrical is applied to all those grains which are 

 more or less of an elongated form, are somewhat triangular, 

 and taper towards each extremity, which exhibit a furrow 

 along their upper surface, and are furnished with three pollen 

 tubes ; while the word elliptical is used to designate those 

 granules which agreeing somewhat with the former in their 

 lengthened shape, having also a line upon their surface, yet 

 possess but one pollen tube. 



On the application of any fluid to pollen of an elongated 

 form, of less density than the fovilla contained in the cells, a 

 remarkable alteration of shape takes place with great quick- 

 ness. Each grain of pollen, prior to the contact of the 

 liquid, lies on the object-glass of the microscope with its long 

 diameter placed horizontally ; immediately on its touching it 

 the position of the long diameter is changed, it becomes ver- 

 tical, and the pollen grain, losing in length, dilates and becomes 

 of a triangular or circular form ; and this change of form, 

 which is here produced by the direct application of the water, 

 occurs also naturally previous to the* emission of the pollen 

 tubes, arising partly from the absorption of fluid from the 

 tissue of the anther, but mainly from the imbibition of the 

 stigmatic secretion. But if a fluid of greater density than 

 that within the cell be applied, no such change is seen to 

 occur, a contraction of the pollen grain and an exudation of a 

 portion of its contents taking place. 



This alteration of form, in both instances, is very satisfac- 

 torily explained by a reference to the principle of end osmosis 

 and exosmosis, which is most beautifully exemplified in the 

 interesting though minute subjects of the present inquiry, 

 and all the varied phenomena of which are referred by Du- 

 trochet to capillary attraction. 



Each pollen grain consists of a turbid fluid called fovilla, 

 containing numerous active molecular particles ; and this fluid 

 is enclosed, according to most observers, in at least two mem- 

 branes, the outer being called extine, the inner intine, and 

 this is of a highly extensible nature. The only exception to 

 this, according to Mohl, is the pollen of Asclepiadaceous 



