INFLUENCE OF WATER ON LAND-PLANTS 41 
4. The Leaf is thickened either by the increase of the 
palisade-tissue, or by the development of special: water- 
storing cells. The former is strongly developed in sun- 
leaves (Fig. 22), the latter tissue is found in succulents. 
The storage of water in fleshy or succulent organs is very 
common in desert and strand plants (see p. 277). All 
parts of the plant exposed to the air may become succulent 
—e.g., the leaves in stonecrop (Fig. 7), the stems in glass- 
wort (Fig. 10). The water is stored in a special tissue, the 
cells of which are large and devoid of chlorophyll ; the 
cell-sap is abundant, clear, but somewhat slimy through 
the presence of mucilage. The presence of mucilage in 
Fic. 9.—TRANSVERSE SECTION oF ROLLED Lear oF Erica cinerea. 
(Hiaguity MaGnirrep.) 
a. cuticle ; 6, epidermis ; c, mucilage in the cells; d, chlorophyll-tissue ; 
e, vascular bundle ; /, air-space ; g, stoma ; h, hair. 
water makes its evaporation difficult, and this difficulty 
is increased by the absence of air-spaces. Lignified 
tissue in succulents is also poorly developed, and there 
is little cork, the retention of water within the plant being 
secured by other means. 
5. The Augmentation of Lignified and Corky Tissues. 
—This serves in leaves the same end as succulence— 
namely, the retention of water within the plant. No 
large reserves of water are here stored away for future 
use; in fact, the actual water-containing and water- 
conducting tissue is small; but what there is, is protected 
by masses of sclerenchyma—elongated cells with thick, 
