ORGANS OF REPRODUCTION. 



261 



form a compound body consisting of four pollen-grains, as 

 in Periploca grceca (Jig. 548); or if the membranes of two 

 or more united parent cells are also incompletely absorbed, 

 we may have a mass consisting of eight pollen-grains, as 

 in Inga anomala {fig. 549), or of some multiple of four, 

 as in many Acacias {fig. 1 2). By some botanists, the union 

 of the pollen-grains in the above instances, is referred simply 

 to the viscid secretion left in such cases by the solution 

 of the parent cells, but their regular an-angeraent in fours or 

 multiples of four, would seem to prove that the special mother 

 cells are more particularly concerned in the formation of such 

 compound pollen-grains, in the manner just described. In the 

 Onagrariacere, the pollen-grains 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 Orchidacere {fig. 550), the pollen-grains cohere in a re- 

 markable degree and form pollen masses which are commonly of a 

 waxy nature, to which the name of pollinia has been given. In 

 the Asclepiadaceffi somewhat similar masses occur (^^. 551); in 



Fig. 550. Fig. 551. 



Fig. 550. Pollinia, p, of Orchis with their caudicles, c, and the retinacula, r, 



at the base. Fig.rt.il. Pistil of Asclepias, witli tlie pollinia.^), 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. 



the latter however, the whole pollinia is invested by a special cel- 

 lular covering. By a careful examination of these pollinia, we find 

 that they are formed of compound masses agglutinated together, 

 and when separated, each of these is found to consist of four 

 pollen-grains. In the pollinia of the Orchidacete we also find 

 other peculiarities; thus, each is prolonged downwards in the 

 s 3 



