124 



anther is in contact with the stigma at the time of flowering 

 and the dried remains of the two may be found together at 

 tip of the mature seed. 



Poa annua is common in many parts of Europe, but is 

 supposed to have been introduced into the United States in 

 recent times. Poa chapmaniana is thought to be a native of 

 the United States. Practically nothing of the phylogenetic 

 history of either is known, but the similarity of the two suggests 



Fig. 1, spikelet, and Fig. 2, stamen of Poa annua. Fig. 3, stamen, and 

 Fig. 4, spikelet, of Poa chapmaniana. Magnification: spikelets, x 123^; 

 stamens, x 25. 



close relationship. It is probably not too rash to formulate 

 the hypothesis that cleistogamy has here been a mechanism 

 of evolution. In thinking of the formation of a new species 

 from a pre-existing stock by any kind of mutative or selective 

 process, we recognize the importance of barriers which prevent 

 the blending of the two strains by hybridization. These barriers 

 may be environmental, or they may take the form of anatomical 

 or physical peculiarities of the organism itself. If we regard 

 P. chapmaniana as having arisen as an offshoot from P. annua 

 we have in cleistogamy a barrier more effective than a mountain 

 range or a sea in keeping the new species from hybridizing 

 with its parent. 



Indiana University 

 Bloomington, Ind. 



