18 THE SEEDS OF THE BLUEGRASSES. 



Poor!}' cleaned samples are apt to contain man}' sterile florets. 

 These are slender, sometimes shrunken, and usually lighter colored 

 than the grain-bearing- florets, which are comparatively plump and 



often dark colored, owing to the color of 

 the grain appearing through the thin palea. 

 (Fig. 3.) _ ^ 



The recognition of the several species of Poa, 

 when the identit}^ is questionable, requires the 

 use of a good lens and a knowledge of the 

 principal distinguishing characters. A sample 

 FIG. 3.-sLds of Mcky ""^er examination should be spread thinly on 

 biuegmss (Poa pratensis): a. a slicet of paper, or, better still, on a black sur- 



grain bearing; 6, .sterile. ^.^^^_ ^y-^.^^ ^ ^^^^ jjg.j^^ ^^^^^ ^^^^^^^ ^^^. ^^^^.^^ 



ing the seeds over while under the lens, they can easily be examined 

 with reference to size, color, distinctness of veins, character of pubes- 

 cence, the condition of the margins of the glume, etc. Exposing the 

 difl'erent sides of the florets to the light while under examination is 

 often absolutely essential in determining the nature of the veins and 

 pubescence. . 



KEY TO THE SEEDS OF THE MORE COMMON SPECIES OF POA AS FOUND 



ON HERBARIUM SPECIMENS. 

 Basal web present. 



Web very persistent and conspicuous _ P. urachnijera. 



Web easily removed, small; keel of the glume pubescent. 

 Intermediate veins distinct. 

 Intermediate veins sharply defined as narrow ridges; glume margins narrow, 

 not easily seen from the side; marginal veins usually smooth. P. trivialis. 

 Intermediate veins usually not sharply defined; glume margins broader, easily 

 seen from the side in fertile florets; marginal veins pubescent. P. 2iratensis. 

 Intermediate veins indistinct. 

 Eachilla segment smooth or nearly so; florets 2-2j mm. long. 



Florets usually broader ab"ove than below the middle; apex usually flaring; 



rachilla segment smooth P. ccnnpressa. 



Florets not evidently broader above than below the middle; apex usually 



golden yellow; rachilla segment sometimes rough P. triflora. 



Rachilla segment usually pubescent. 



Florets 25-8 mm. long, usually not yellow at the apex P. nemoralis. 



Basal web not present. 

 Florets strongly pubescent. 



Intermediate veins distinct; palea keels prominent, often arched forward. 



P. a)inua. 



Intermediate veins indistinct; palea keels not arched P. alpina. 



Florets not pubescent J', sudetiat. 



