TORREYA 



Vol. 29 Q . K n . K N"- 5 



beptember- October 



Cleistogamy In Poa Chapmaniana 

 Paul Weatherwax' 



It is usually difficult for a beginner in the study of grasses 

 to distinguish between Poa annua L. and Poa chapmaniana 

 Scribner. As in so many other instances, the differences be- 

 tween the two are clear after a few specimens of each have 

 been seen, but it is difficult to visualize the characteristics 

 from the descriptions. In the manuals in general use in the 

 eastern part of the United States, and in many of the state 

 floras, the keys use the presence or absence of cobwebby hairs 

 at the base of the lemma and the prominence of the inter- 

 mediate nerves of the lemma as distinguishing characters; 

 but both prove confusing in practice, and the additional charac- 

 terizations given in the descriptions are little more illuminating. 



Some time ago it was pointed out to the writer by C. C. 

 Deam that the anthers of his specimens of P. chapmaniatia 

 were only 0.2 mm. long, while other Indiana species of Poa 

 had anthers much longer than this. Further investigation 

 following this lead, showed that the difference between the 

 two conditions is much more fundamental than merely length 

 of anther. Poa chapmaniana is completely cleistogamous. 



Differences between the spikelets of the two species may 

 be seen in Figures 1 to 4. The flower of P. annua has large 

 feathery stigmas, and three well-developed anthers, (Figs. 1 

 and 2.) at least 1 mm. long, and exserted on long, slender 

 filaments at anthesis. Pollination is doubtless accomplished 

 by contact and by the wind. The flower of P. chapmaniana 

 has small, poorly developed stigmas, and a single stamen, 

 whose anther (Fig. 3) is only 0.1 to 0.2 mm. long and produces 

 only about 16 to 24 pollen grains. The floret does not open 

 and self-pollination within the floret necessarily occurs. The 



1 Publication No. 37 of the Waterman Institute for Scientific Research, 

 Indiana University. 



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