I30 THE FLOWERING PLANT. 



sessile anther upon it. Each lobe of the anther contains an 

 agglutinated club-shaped pollen mass or pollinium. The stalks of 

 the two pollinia slope down to a little viscid knob, the rostellum, 

 below which is a broad sticky stigma. A bee alighting on the 

 labellum, for the purpose of piercing the tissue of the spur and 

 licking up the sweet sap (nectar is absent), is sure to strike the 

 rostellum, which becomes detached, and draws with it the pol- 

 linia. The bee leaves the flower with these structures attached 

 to its head like a pair of horns. They soon droop forwards, and 

 are likely to strike the stigma of the next blossom visited, these 

 remaining in whole or part. 



Some American flowers are bird-pollinated, their visitors being 

 humming-birds, and in some cases small insects are not excluded, 

 as they attract these larger useful visitors. It is also believed 

 that many Cape flowers, excreting as they do large quantities of 

 nectar, are specially suited for the visits of small birds. 



Self-Pollination. — The fact that the majority of flowers are 

 bisexual leads one to suspect that this process occurs not unfre- 

 quently. Many forms which lay themselves out for cross-pollina- 

 tion provide for the other form as a last resort. Thus, in the 

 forget-me-not, the stigma at first protrudes from the flower, but 

 later on the corolla tube elongates, and brings its circlet of sessile 

 anthers to the same level. Again, in Composites, the branches 

 of the stigma, if not cross-pollinated, sometimes curl round and 

 touch the top of the anthers with their receptive inner surfaces. 

 This curving regularly takes place in certain small self-pollinated 

 Composites, as groundsel. 



Little specialized forms, like buttercup and rose, must often be 

 self-pollinated, and the only perfect bars are complete dichogamy 

 and self- sterility. 



Comparison of several closely allied flowers often bring out the 

 fact that the smaller ones are self-pollinated (cf p. 120). Com- 

 pare, for instance, the large white flowers of stitchwort with the 

 small ones of chickweed. Regularly self-pollinated flowers are 

 characterized by inconspicuousness, partly due to their small size 

 generally as a whole, and still more to the minuteness of the 

 petals, which are white or pale and devoid of honey-guides. 

 Scent and nectar are practically absent, and the stigmas are so 

 placed that pollen can easily reach them from the anthers of the 

 same flower. The stamens are often few in number, and pro- 

 duce comparatively little pollen. 



All this is carried to the extreme in cleistogamous flowers, i.e., 

 minute self- pollinating ones, which never open, and exist in 

 addition to ordinary ones. The best example is dog-violet. In 

 summer the ripe fruit of the cross- pollinated flowers will be found, 



