130 THE FLOWERING PLANT. 
sessile anther upon it. Hach 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 rostel/um, 
below which is a broad sticky sttgma. <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, 7.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, 
