POLLINATION AND FERTILIZATION 141 
self-fertilization, for several generations at any rate, produces 
better results than cross-fertilization.! 
Whenever cross-pollination by the wind or by the agency 
of animals is impossible, it is evident that self-pollination 
would be advantageous, since it is infinitely better than no 
pollination at all. Many highly successful weeds owe their 
predominance partly to the 
fact that they produce good 
seed after self-pollination. 
Since cross-fertilization at 
intervals appears to be suffi- 
cient to keep up the strength 
and fertility of many kinds 
of plants, there might be some 
advantage in uniting the cer- 
tainty which characterizes self- 
pollmation with the renewal 
of strength which comes from 
cross-pollination. Violets and 
many other less familiar plants 
unite the two methods by pro- 
ducing ordinary showy flowers 
and also inconspicuous closed, 
or cleistogamous, flowers. In 
violets the latter are borne 
on flower stalks close to the 
Fig. 124. A violet with cleistogamous 
flowers as seen in late July or early 
August, after the conspicuous flowers 
have disappeared 
cl, cleistogamous flowers; caps, cap- 
sules produced by earlier flowers of 
the same sort 
ground (fig. 12+) and usually, 
before maturing, become partially buried in the earth. Pol- 
lination occurs within the closed flower, the pollen tubes 
developing within the anthers and making their way to the 
stigma. The cleistogamous flowers produce many more seeds 
than the showy ones, but the latter insure at least occasional 
cross-pollination since they are freely visited by bees and 
other flying insects. 
1 See ‘Tobacco Breeding,” Bulletin 96, Bureau of Plant Industry, U.S. 
Dept. Agr., 1907. 
