78 

 Agrostis. |Alopecurus geniculatus 



Note 4. The height of the ribs may be roughly deter- 

 mined by holding the blade up to the light ; the 

 amount of contrast between rib and furrow measures 

 the height of rib. For example : 



Aira. Ribs dark, furrows pure white. 



Alopecwus geniculatus . Ribs dark, furrows yellow. The 

 contrast is strong, and the ribs therefore very high. 



Agrostis. Little contrast between rib and furrow, therefore 

 the ribs are comparatively low. 



Note 5. On the same blade the mid-rib may be 

 different from the lateral ribs ; Festuca elatior 

 (XIV. 15) for example, has a flat mid-rib. 



In Psamma^ adjoining lateral ribs differ in height and shape ; 

 these are described as mixed ribs (XIV. 2). The same 

 condition is a striking feature of the hairy Koeleria. 



