CHAP. 11. POLLEN. 155 



the whole cavity of a cell of the anther, as in Asclepiadese. 

 But the most curious instances of the cohesion of the grains 

 of pollen is to be found in Orchideae ; in which some genera 

 have the pollen in its common pulverulent state, with no 

 remains of the cellular substance in which it was developed ; 

 others have the granules held together by some of the cellular 

 substance in an elastic state, and forming a distinct appendage 

 to the pollen called the caudicula; while othei's have the grains 

 united either by threes or fours, or in wedge-shaped masses, 

 or in a hard, dry, solid body. It appears from Mr. Fran- 

 cis Bauer's observations, that the masses of pollen of both 

 Asclepiadeae and Orchideae, in the most solid state are really 

 cellular, the grains of pollen being contained in cavities, the 

 walls of which are either separable from each other as in some 

 Orchideae, or are ruptured without a separation of the cavities 

 as in Asciepiadeae. {See the Observations on Orchidece and 

 Asclepiadece before referred to.) And this is quite in con- 

 formity with Mirbel's account of the origin of the pollen in 

 the Gourd. 



The granules are generally discharged at once, upon the 

 dehiscence of the anther, or at least are at that time wholly 

 formed. But in some Aroideae, which emit their pollen by a 

 hole in the apex of their anther, the fomiation or develope- 

 ment of pollen must be going on for a considerable time after 

 the first emission. A single anther continues to secrete and 

 discharge pollen, till, as Brown remarks, the whole quan- 

 tity produced greatly exceeds the size of the secreting organ. 



The surface of the pollen is commonly smooth. In some 

 plants it is hispid, as in the Gourd and Ipomaea purpurea ; in 

 others it is covered with strong points, as Hibiscus syriacus ; 

 and in all cases, when there are asperites of the surface or 

 angles in its outline, it is asserted by Guillemin to have a 

 mucous surface, which was first observed in Proteaceas by 

 Brown. 



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 Anethuni 

 segetura, and in a very remarkable degree in Tradescantia 

 virginica, where the grains become curved. In Colutea arbor- 



