312 



BOTANY 



convey the pollen to the receptive stigmas of younger flowers. In the Orchidaceae 

 and Asclepiadaceae self-pollination is rendered impossible both by the nature of 



tlie pollen masses and by their position. 

 A complicated form of structural 

 contrivance, by means of which cross- 

 pollination is secured, may be seen in 

 a flower of Salvia pratensis (Fig. 244). 

 The anthers of this flower are concealed 

 in the upper lip of the corolla, from 

 which the style, with its bilobed 

 stigma, projects. When a bumble-bee 

 visits the flower in search of honey, 

 it must first with its proboscis push 

 out of the way the small plate (s), 

 formed of two sterile anther halves 

 grown together. These are situated 

 at the ends of the short arms of the 

 connectives (c), which are so elongated 

 that they might easily be mistaken 

 for the filaments (/) of the stamens. 

 The fertile -anther halves are situated 

 at the other ends of the connectives, 

 and are thus brought in contact with 

 Fio. 243. -Flowers of Aristolochia dematitis cut the hairy back of the bumble-bee when 

 through longitudinally. /, Young flower in which it pushes against the plate at the short 

 the stigma (N) is receptive and the stamens (S) e nds of the lever-like connectives. The 

 have not yet opened ; II, Older flower with the Ren t , mg attached to the bee will 

 stamens opened, the stigma withered, and the , r , . . . . , . , 



hairs on the coroila dried up. (x _>.) be brushed oil its back by the forked 



stigma borne on the elongated style 



of an older flower (Fig. 244 //). Good examples of hercogamous flowers are afforded 

 by the Papilonaceae, by Kalmia, whose anthers are held in pockets of the corolla 

 and are only released when touched by a visiting insect (Fig. 245), by Vincu, etc. 



Fio. 244. Pollination of Salvia pratensis. 1, flower visited l.y a l,unil>le-l>ee, showing tho projec- 

 tion of the curved connective from the helmet-shaped upper lip, and the de]>ositi<m <>t !] 

 pollen on the back of the bumble-bee; 2, older flower, with connective drawn Uirk, and 

 elongated style ; 4, the staminal apparatus at rest, with connective enclosed within the upper 

 lip ; 3, the same, when disturbed by the entrance of the proboscis of the bee in the direction of 

 the arrow ; /, filament ; e, connective ; s, the obstructing halt of the anther. 



Some plants bear in addition to hermaphrodite flowers others that are purely 

 female. They are termed polygamous, and the distinction is made between 



