MORE ABOUT POLLINATION 



321 



Sorrel (Oxalis) and touch-me-not (Impatiens) are ex- 

 amples of plants which sometimes produce cleistogamous 

 flowers. Low temperature is beheved to be the principal 

 cause which induces their pro- 

 duction. Lamium, one of the 

 mint family, produces cleistog- 

 amous flowers in spring and 

 autumn, and open flowers in 

 the summer. 



Fig. 141. — One of the milkworts 

 {Poly gala polygama), bearing both 

 aerial open flowers and subterra- 

 nean cleistogamous flowers. The 

 subterranean flowers are self-pol- 

 linated and produce pods which 

 contain many seeds. 



H. Which Kind of Pollina- 

 tion is Better ? — The test of 

 pollination is the quality of 

 the progeny resulting from the 

 seeds produced thereby. It is 

 commonly beheved that the 

 progeny resulting from cross- 

 poUination is in general bet- 

 ter than that resulting from 

 close-poUination. Numerous 

 experiments, conducted 

 through many years, tend to show this to be true. But 

 there is something to be said in favor of the other method. 

 In some plants, close-polhnation has been shown to result 

 in progeny of at least equal vigor with that resulting from 

 cross-pollination. Tobacco, petunia, and California poppy 

 are examples of this. 



It is evident that close-pollination is much simpler than 

 cross-polHnation. It does not require such elaborate 

 structures. It does not involve any waste of pollen. 

 (Some cleistogamous flowers have only one anther and 

 only a few grains of pollen in that.) It is certain, not 



