MORPHOLOGY OF THE VEGETATIVE ORGANS 109 



decreases. This rolling is a normal protective process and 

 should not be confused with wilting. The blades of roll- 

 leaves are marked on one or both surfaces with parallel 

 channels and ridges, the latter being the nerves. The 

 tissues of the ridges are firm and resistant while the cells 

 of the channels are larger and thin-walled. These large 

 cells, called bulliform cells, readily give off moisture. 

 When the air is moist they remain turgescent, thus push- 

 ing the ridges apart and holding the surface flat or at 

 least partly open. As the air grows drier the bulliform 

 cells become flaccid and the blade closes or rolls. To 

 further aid the blades of xerophytic grasses to resist too 

 great a loss of water, the stomata are usually arranged 

 along the sides of the channels and are not exposed when 

 the blade rolls. 



Examples of roll-leaves are forms of Festuca ovina and species of 

 Stipa and Spartina. 



SCALES AND BRACTS 



140. Scales are the reduced leaves found upon shoots 

 below the foliage-leaves. They may be observed near 

 the base of shoots of all perennial grasses, occupying gen- 

 erally the portion below the surface of the soil and often 

 a portion for a short distance above the soil. These scales, 

 by overlapping above the growing point as a bud, protect 

 the shoot as it pushes through the ground. Usually there 

 is a gradual transition from scales to foliage-leaves, but 

 sometimes the change is abrupt. In large grasses such as 

 Gynerium sagittatum, the portion of the culm occupied by 

 scales is much greater and may extend several feet above 

 the surface of the soil. Bamboos, especially the vigorous 

 shoots of the large species, furnish excellent examples of 



