STRUCTURE.] j FORMS OF POLLEN-CHAINS. 357 



with crested angles ; among it occur some of the most beauti- 

 ful microscopical appearances in the vegetable kingdom. In 

 all cases, where there are asperities of the surface or angles 

 in the outline, pollen is asserted by Guillemin to have a 

 mucous surface, which was first observed in Proteads by 

 Brown. But Mohl finds that the presence of mucosity upon 

 pollen is a constant character, at least when the grains first 

 quit the anther ; and that a power of secreting a viscid sub- 

 stance is one of their functions when perfectly smooth, as 

 well as when covered with points and inequalities. He, 

 however, admits that hispid pollen is generally more viscid 

 than that which is smooth. 



The figure of the granules is various ; most frequently it is 

 spherical or slightly oblong. Many other forms have, how- 

 ever, been described. The cylindrical exists in Anethum 

 segetum, and in a very remarkable degree in Spiderwort 

 (Tradescantia virginica,) where the grains become curved : in 

 Bladder Senna (Colutea arborescens), they were observed by 

 Guillemin to be nearly square ; in Lavatera acerifolia to be 

 oval, much attenuated to each end ; in CEnothera they are 

 triangular, with the angles so much dilated as to give the 

 sides a curved form ; in Jacaranda tomentosa I have remarked 

 them to be spherical, with three projecting ribs tapering to 

 either apex ; in some Composites (Cichoraceae) the granules are 

 spherical with facettes; in Teazelworts (Dipsacacese) a de- 

 pressed polyedron; in Scabiosa caucasica, patelliform and 

 angular. In numerous plants it is oval, with a furrow on 

 one side, like a grain of wheat; in Thunbergia fragrans, 

 Mimulus moschatus, &c., it is strongly ribbed, as if formed 

 of many folds ; in Morina persica, cylindrical, with a narrow 

 neck rising abruptly from each side ; in Scolymus, Scorzo- 

 nera, &c., it is a polygon, with crested angles ; and of all 

 these there are numerous modifications, some of which are 

 represented in Plate IV. See Elements of Botany , p. 84, fig. 

 173, for numerous figures of pollen-grains. 



In consequence of the great diversity of forms observable 

 in pollen, it has been supposed that it might be employed in 

 the definitions required for systematic botany ; and Messrs. 

 Guillemin and Adolphe Brongniart have stated that plants of 



