7 
POLLINATION 55 
to the sun; whilst in wet weather they hang down, and the 
rain cannot so easily enter them. The Herb Robert is an 
example of this. 
Self- Within the last thirty years numerous instances 
Pollination. have been collected of flowers which pollinate 
themselves. Kerner, in his “ Natural History of Plants,” 
enumerates many hundreds, and shows that there is a certain 
relation between the altitude at which a plant is found and 
the frequency of self-pollination, no doubt owing to the com- 
parative scarcity of insects at high levels. Cleistogamous 
flowers—those which never open (Gk. cleistos = closed)—must 
invariably pollinate themselves. The Violet, Wood-sorrel, 
many Chickweeds, many of the Leguminose have these 
cleistogamous flowers, which are formed in late summer. 
-Many of the Cruciferee have small, white, inconspicuous 
flowers which pollinate themselves: the anthers of the four 
long stamens slowly revolve until they are over. the stigma, 
then they open and the pollen falls on the stigma. Groundsel 
generally pollinates itself, and many other instances might be 
evel. = 
The interesting thing about all these flowers is that, although 
they are usually self-pollinated, insect visits have been recorded, 

Fics. 67 AND 68.--STELLARIA GRAMINEA. 
67.—First STAGE, SHOWING OUTER ANTHERS COVERED WITH POLLEN. 
68.—LATER STAGE, STIGMAS SPREADING. 
and they are apparently cross-pollinated at intervals. Even 
those plants which have cleistogamous flowers form, earlier in 
the season, other flowers which are cross-pollinated. And in 
