90 TEXTBOOK OF BOTANY 



Fig. 47. Pulvinus and section of pulvinus from leaf of sensitive plant, both en- 

 larged. When the leaf is touched, the water in the cells on side A passes outward 

 into the intercellular spaces, causing the cells partially to collapse. The pressure 

 of the cells on side B then forces the leaf downward. Courtesy World Book Co. 



Rolling of leaves. Leaves of many grasses, especially those that live in 

 dry regions, have chloroplasts in part of the mesophvll tissue only. 

 There are other parenchyma cells, called reservoir cells, in v^hich water 

 noticeably accumulates. When the plants are subjected to drought these 

 cells lose water and the leaves become folded, or rolled. Fig. 48 illus- 

 trates sections of leaves with reservoir cells. The inward rolling of 

 leaves of corn and beach grass, and the folding of bluegrass leaves are 

 common examples. These leaf movements have often been cited as ex- 

 amples of purposeful behavior, on the assumption that the leaves always 

 roll so that the stomates will be on the inside. In nature, however, one 

 may find leaves rolled in such a way that the stomates are on the out- 

 side and more exposed to light and dry winds than before the rolling. 

 Bluegrass leaves which fold upward in drought have more of the 

 stomates on the upper surface, whereas corn leaves which roll up have 

 a greater number of stomates on the lower, exposed surface. 



Aquatics. The leaves of submerged plants are noted for their thinness. 

 They are usually ribbon-like as in pondweeds, or branched and dissected 

 as in the water milfoil, water crowfoot, and water marigold. The leaf 

 blades of many aquatic plants float on the surface of the water. The 

 most striking features of floating leaves are the large air cavities and the 

 restriction of stomates to the upper surface. However, paired guard cells 

 often occur on the lower surfaces, and when the leaves develop above 



