320 



FLOWERS 



ably pollinate low stigmas. Usually, in such plants, pollen 

 from low anthers will germinate only on low stigmas, and 

 that from high anthers only on high stigmas. And the 

 two kinds of flowers are usually borne on different individ- 

 uals. Thus we have three devices of the same kind of 

 plant, all tending to prevent close-pollination and to pro- 

 duce xenogamy. 



Fig. 140. — Flowers of the Chinese primrose, illustrating dimorphism. Note that 

 that part of the insect which strikes the anthers of one flower will be likely to strike 

 the stigma of the other. 



G. Cleistogamy. — Some plants produce flowers which 

 never open and are necessarily self-pollinated. Such 

 flowers are called deistogamous. The violet is the most 

 familiar plant which regularly produces deistogamous 

 flowers. They are produced some weeks or even months 

 after the open, colored ones. They are colorless and are 

 borne on short stalks, very near the ground. Often they 

 are actually subterranean, the seeds which they produce 

 thus being in a position which is very favorable for their 

 germination. One of the milkworts (Polygala polygama) 

 also regularly produces deistogamous flowers and these are 

 distinctly subterranean. (See Figure 141.) 



