ORGANS OF BEPEODrCTIOK. 



253 



Fiq. 549. 



consisting of eight pollen-cells, as in Inga anomala (fig. 547), or 

 of some multiple of four, as in many species of Acacia {fig. 

 549). In the Onagracese, the pollen-cells are loosely connected 

 by long viscid filaments or threads, -which seem in this case 

 to be wholly derived from a secretion left by the solution of 

 the parent cells. In the Orchidacepe, the pollen-cells cohere in 

 a remarkable degree and form pollen-masses which are com- 

 monly of a waxy nature, to which the name of ipol- 

 linia has been given {fig. 650, p). In the Ascle- 

 piadacese somewhat similar masses occur {fig. 551) ; in 

 the latter, however, the whole sm-face of each pollen- 

 mass is invested by a special cellular covering. By a 

 careful examination of these pollinia, we find that 

 they are formed of compound masses agglutinated to- j"/^. 549. Mass 

 gether, and when separated, each of these masses is of spuericai 

 found to consist of four pollen-cells. In the pollinia from"a%e- 

 of the Orchidacese we also find other peculiarities; aes of Aca- 

 thus, each is prolonged downwards in the form of a 

 stalk called the caiidicle {fig. 550, c), which adheres commonly 

 at the period of dehiscence to one or two little glandular masses 



Fig. 550. 



Fig. 552. 



Fig.haO. Pollinia, p, of Orchis with their caudicles, r, and the retinacula, 



r,r, at the base. Fig. 551. Pistil of a species of Asclepias, with the 



pollinia, jt, adhering to the stigma, s. h. Pollen-masses separated. 



Fig. 552. Upper part of the flower of an Orchis, showing the pollinia 

 adhering to the stigma by the retinacula, a. 



called retinacula {figs. 552, a, and 550, r), which are placed 

 on the upper surface of a little projection called the rostellum 

 situated at the base of the anther. 



We must now return to the more particular description of the 

 pollen-cell, or pollen-grain as it is also more frequently called. 

 We shall treat of it under four heads, viz. : — 1. Its Wall or Coats ; 

 2. Its Contents ; 3. Its Form and Size ; and 4. Its Dehiscence. 



