356 POLLEN OF ORCHIDS AND ASCLEPIADS. [BOOK r. 



waxy pollen mass may be found embedded in each division of 

 the anther. At a somewhat later period we may see the four 

 cells in Picea and Abies, in which the four pollen-grains lie 

 closely united ; and it offers a pleasing spectacle under the 

 microscope to observe each grain expand itself by the absorp- 

 tion of water until it bursts its case in order to escape, leaving 

 the four cells emptied of their contents." 



Link supposes the cellular substance in which pollen is 

 generated to be semiorganic, and calls it collenchyma, con- 

 sidering with me that it is what forms the appendage of the 

 pollen masses of Orchids. But it can hardly be called semi- 

 organic, especially if it is examined in Polystachya ramulosa. 



It also appears from Mr. Francis Bauer's observations, 

 that the masses of pollen of both Asclepiads and Orchids in 

 the most solid state, are truly cellular, the grains of pollen 

 being contained in cavities, the walls of which are either 

 separable from each other, as in some Orchids, or are rup- 

 tured without a separation of the cavities, as in Asclepiads. 

 (See the Observations on Orchidacece and Asclepiadacece, 

 before referred to.) It does not, however, follow that because 

 pollen is engendered in the interior of cells, its grains must 

 therefore adhere originally by an umbilicus ; and in fact the 

 part so described by Turpin has no existence. 



When the pollen-grains are fully formed they are generally 

 discharged at once, upon the dehiscence of the anther. But 

 in some Arads, which emit their pollen by a hole in the apex 

 of their anther, the formation or development 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 quantity 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 Ipomoea purpurea ; in 

 others it is covered with strong points, as the Altha3a frutex 

 (Hibiscus syriacus) ; in Jatropha pandursefolia it is granular ; 

 in many Labiates, banded; in Passiflora, reticulated; in Gera- 

 nium sylvaticum, crested; in Armeria vulgaris, polygonal, 



