26 KEELED AND HAIRY GRASSES [CH. 



In 'some grasses the tissue over the mid-rib is con- 

 siderably raised and strengthened on the dorsal side of the 

 blade as a " keel." 



Keel more or less prominent. 



xirrhenathenim (sheath keeled). 



Poa (all except. P. maritiraa). 



Dactylis. 



Bromus. 



Bromtis asper (sheath keeled, often a white line). 



Holcui laiuxtus (slight and decurrent) (Fig. 15). 



Digraphis (Fig. 14). 



Glyceria. 



Most grasses are glabrous, but there are a number in 

 which hairs are nearl 3^ always a prominent feature. It 

 must be remarked, ho\<rever, that with grasses, as with 

 other plants, the character of pubescence is apt to vary 

 with the situation. In general it may be stated that a 

 hairy grass tends to become more glabrous in a moist 

 situation, and more pubescent in a dry one, but the rule 

 is by no means absolute. In some cases, — e.g. Avena 

 pubescens, A. flavescens, Agropyrum, the hairs are almost 

 entirely confined to the crests of the ridges (Figs. 10, 15). 

 The following is a list of hairy grasses. 



Hairy Grasses. 



Holcus (Fig. 15). Hordeum. 



Molinia aerulea. Anthoxanthum. 



Brachypodmm sylvaticum. Avena flavescens (Fig. 10). 



Agropyi-um (variable). A. ptcbescens. 



Brormis asper. Triodia. 



B. mollis. Kceleria. 



To a less extent. 

 Festiica sciuroides (on ribs). Meliea. 



