IV] VASCULAR BUNDLES 67 



Bristles — i.e. long, sharp, stiff hairs — are not very 

 common. They occur on Nardus, Anihoxanthum Puelii, 

 Panicum, Cynodon. 



PapillsB occur on the leaves of Glyceria, Nardus, 

 Leersia, &c. 



Poa pratensis has soft hairs on the upper epidermis. 



The stomata of Sesleria ccBrulea are depressed and 

 six-celled, two guard-cells being overgrown by four ac- 

 cessory cells, but in most grasses they are of the ordinary 

 type with two elongated guard-cells only. 



As regards the vascular bundles constituting the 

 venation, they are as is well known parallel from base 

 to apex in our common grasses, with linear leaves, and 



Fig. 25. Transverse section of part of leaf of Elymus arenarius, partly 

 inroUed ( x about 30), showing ridges of unequal height, of which 

 the higher are flat above. Vascular bundles girdered, the stronger 

 above and below. Motor-cells in each groove cause the inroUing of 

 the lamina by their contraction. 



are usually of four orders as regards strength. Those of 

 the first (e.g. mid-rib) and second orders have conspicuous 

 vessels, but those of the third and fourth orders may be 

 practically devoid of vessels, though xylem and phloem 



5—2 



