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 
