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 



FIG. 9. TRANSVERSE SECTION OF ROLLED LEAF OF Erica cinerea. 

 (HIGHLY MAGNIFIED.) 



a , cuticle ; &, 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 

 Dy masses of sclerenchyma elongated cells with thick, 



