II.] FORM AND QUANTITY OF POLLEN. 39 



In Dimorphous species the pollen of the short- 

 styled form is generally larger than that of the long- 

 styled form, but in Linum, according to Hildebrand, 

 (" Die Ges. Verth. bei den Pflanzen," p. 37) it is of the 

 same size in both forms. 



In Faramea, another Dimorphous group, the sur- 

 face of the pollen grains is different in the two forms 

 (Thom^ " Das Gesetz der vermiedenen SeLbstbe- 

 fruchtung bei den hoheren Pflanzen," 1870), the 

 smaller grains of the long-styled form being smooth, 

 while those of the short-styled form are studded 

 with small points ; in consequence of which the pol- 

 len-grains are less easily detached from the anther ; 

 this difference possibly has reference to the different 

 position of the two forms ; the smooth ones being 

 sheltered by the flower ; while the larger pollen- 

 gra,ins, which are produced in the anthers of the 

 long stamens, and are therefore more exposed to the 

 wind, are, in consequence of their roughness, less 

 liable to be blown away and wasted. 



According to D. Muller {Bot. Zeit., 1857) the pollen 

 of the small flowers of Viola elatior and V. lancifolia 

 is minute and round. Herr von Mohl, however, found 

 no difference between the pollen of the large and 

 small flowers in V. mirabilis {Bot. Zeit., 1863), The- 

 number of grains in these flowers is very small. So 

 also in the cleistogamous flowers of Oxalis acetosella, 

 there are not above two dozen pollen-grains in each 

 of the five larger anthers, and one dozen in each of 

 the five smaller ones. The ovules are about twenty 

 in number. 



It is interesting to notice that the contrivances by 



