48 



FIRST BOOK OF GRASSES 



for the grain and is usually grown fast to it, is free 

 from the grain. In most of the species it is a loose 

 thin sac, which readily tears when moistened, leav- 

 ing the grain naked. In one of our species (Sporobo- 



lus heterolepis) the pericarp is firm, 

 like a thin shell about the grain. 



In Cinna the two nerves of the 

 palea are so close together that 

 they appear to be a single nerve. 

 In one of the species this nerve 

 may be easily split into two, dem- 

 onstrating the derivation of the 

 apparent single nerve. 



In several genera the glumes are 

 enlarged. In timothy (Fig. 38) 

 they are firm in texture, strongly 

 keeled, and abruptly awned, while 

 the lemma is much smaller, thin in 

 texture, and awnless. The spike- 

 lets are so congested on the short 

 branches of the cylindrical spike-like 

 panicle that the beginner may be 

 puzzled to know just what is the 

 unit of inflorescence. Keeping in 

 mind that a spikelet never is com- 

 pound, the student will divide and 

 redivide the cluster until he finds 

 an object having a single pair of 

 glumes containing the floret. Fig. 39. Spikelet of 



. ., , . ~ . . «., , Alopecurus genicu- 



Another modification of the glumes i a t us . 



Fig. 38. Spikelet of 

 Phleum pratense; 

 floret above. 



