CH. i] The Morphology of Grasses 7 



The veins, i.e. vascular bundles surrounded by a sheath of 

 thick-walled tissue run along parallel to each other from the 

 base to the apex of the blade. The thickness of the latter may 

 be almost uniform from edge to edge, but usually the tissue is 

 raised over the vascular bundles and forms the well-known longi- 



Fig. 8. Transverse section of one-half of a blade of Pea pratensis x50. The 

 blade is rible^s above. Below is seen the prominent mid-rib (keel) which 

 is flanked on either side above by a row of motor cells. The motor cells 

 are confined to this area of the blade which therefore tends to fold up 

 under dry conditions. 



tudinal ridges or ribs on the upper surface. These ridges vary 

 much in outline and prominence in different species as is shown 

 by Figs. 911. 



The tops of these ridges often consist of very hard tissue 

 called sclerenchyma which is composed of thick-walled cells. 



Fig. 9. Transverse section of a blade of Alopecurus pratensis x 20. Note the 

 low flat-topped ridges (ribs) with the well-developed motor cells situated 

 in the furrows between. The vascular bundles of the first order are 

 girdered both above and below. 



Continuous longitudinal bands of sclerenchyma also run along 

 the blade close to the lower epidermis. These bands again vary 

 much in relative size and strength with different species, in some 

 being very slight, in others joining up and forming a continuous 

 layer across the lower surface. This sclerenchyma tissue serves 



