140 INTRODUCTION TO BOTANY 
In many kinds of plants self-pollination is entirely effective. 
In others it produces seed which is good but not so abundant 
or so sure to grow into vigorous plants as 
that which is due to cross-pollination. In 
still other plants cross-pollination is abso- 
lutely essential to the production of seeds 
that will grow at all. 
To the fact that self-pollination is in 
many cases not wholly satisfactory, but 
better than none at all, is probably due 
Fic, 122. Stamens the existence of many flowers like the 
and pistils of round- o es 
leaved mallow common dooryard mallow, or “cheeses, 
The flower has been Which has moderately showy petals and is 
open for a consider- often cross-pollinated by insects, but can 
able time, and the : ‘ 
stigmas have curved also pollinate itself by the contact of the 
so as to touch the curving stigmas with the stamens (fig. 122). 
anthers and in this ‘ 
way absolutely toin. Such flowers are sometimes able to secure 
sure self-pollination. insect pollination, but in default of this 
After H. Miller 
they do produce a crop of seeds as a result 
of their self-pollination. 
133. When self-pollination is advantageous; cleistogamous 
flowers. Some flowers are usually self-pollinated except when 
cross-pollinated by accident 
or human agency. Wheat 
is a notable instance of the 
kind, and apparently self- 
pollination can go on in 
this grain fora long period pyq. 123. Facilities for insect-pollination 
without injury to the fer- or self-pollination in flowers of the matri- 
tility or the robustness mG ei CBE 
of the offspring. Experi- In the flower at the left (earlier stage) the 
bed anthers are spread apart and are likely to 
ments in Palsing selected come in contact with insect visitors; in the 
varieties of tobacco seem ‘lower at the right (later stage) the anthers 
: . close together over the stigma, insuring 
to show that in this plant self-pollination. After Knuth 
1 See Wheat: Varieties, Breeding, Cultivation,’ Bulletin 62, Univ. of 
Minn. Agr. Exp. Sta., 1899. 
