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 

 the existence of many flowers like the 

 common dooryard mallow, or " cheeses," 

 which has moderately showy petals and is 

 often cross-pollinated by insects, but can 

 also pollinate itself by the contact of the 

 curving stigmas with the stamens (fig. 122). 

 Such flowers are sometimes able to secure 

 insect pollination, but in default of this 

 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 for a long period 

 without injury to the fer- 

 tility or the robustness 

 of the offspring. 1 Experi- 

 ments in raising selected 

 varieties of tobacco seem 

 to show that in this plant 



FIG. 122. Stamens 

 and pistils of round- 

 leaved mallow 



The flower has been 

 open for a consider- 

 able time, and the 

 stigmas have curved 

 so as to touch the 

 anthers and in this 

 way absolutely to in- 

 sure self-pollination. 

 After H. Muller 



FIG. 123. Facilities for insect-pollination 

 or self-pollination in flowers of the matri- 

 mony vine (Lycium) 



In the flower at the left (earlier stage) the 

 anthers are spread apart and are likely to 

 come in contact with insect visitors ; in the 

 flower at the right (later stage) the anthers 

 close together over the stigma, insuring 

 self-pollination. After Knuth 



1 See ft Wheat : Varieties, Breeding, Cultivation," Bulletin 62, Univ. of 

 Minn. Agr. Exp. Sta., 1899. 



