202 



ESSENTIAL ORGANS. POLLEN. 



and either remain united in fours or multiples of four, as in some 

 Acacias, Periploca greeca (fig. 349), and Inga anomala (fig. 354), or 

 separate into individual grains (fig. 348, 5), which by degrees be- 

 come mature pollen, (figs. 348, 6, 351, 352). In Acacia ringens, there 



are eight pollen-grains united; in Acacia decipiens, twelve; and in 

 Acacia linearis, sixteen. Occasionally the membrane of the parent 

 pollen-cell is not completely absorbed, and traces of it are detected in 

 a viscous matter surrounding the pollen-grains, as in Onagrariacea?. 

 In Orchidaceous plants, the pollen-grains are united into masses or 

 pollinia, by means of viscid matter. In Asclepiadacese (fig. 353), the 

 pollinia, p, seems to have a special cellular covering, 

 derived from a layer of reproductive pollen- cells, 

 or from the endothecium. Pollinia in different 

 plants vary from two to eight. Thus, there are 

 usually two in Orchis, four in Cattleya, and eight in 

 Lselia. The two pollinia in Orchis Morio, accord- 

 ing to Amici, contain each about 200 secondary 

 smaller masses. These small masses, when bruised, 

 divide into grains which are united in fours. In 

 Orchids, each of the pollen-masses has a prolonga- 

 tion or stalk, called a caudicle (cauda, a tail,) which 

 often adheres to a prolongation at the base of the 

 anther, called rostellum (rostellum, a beak), by 

 means of a viscid tenacious matter secreted by cells, and denominated 

 retinacula, (retinaculum, a band or rein). Lindley considers the cau- 

 dicle as derived from the stigma, and not from the pollinary tissue. 

 The term clinandrum (*x/, a bed, and yyp, a stamen), is sometimes 

 applied to the part of the column in Orchids, where the stamens are 

 situated. 



422. When mature, the pollen-grain is a cellular body having an ex- 

 ternal covering, extine (exto, to stand out, or on the outside), and an in- 



Fig. 349. Pollen of Periploca graeca, showing four grains agglutinated together. 

 Fig. 350. Pollen of Inga anomala. The grains united in multiples of four. 

 Fig. 351. Pollen-grain showing the extine covered with small punctuations. 

 Fig. 352. Pollen-grain with the extine covered with granulations. 



Fig. 353. Flower of Asclepias, showing the pollinia or pollen-masses, p, attached to the 

 stigma. 



