124 THE PLANT 
water for the time of extreme drouth. Xerophytic species of Helianthus 
furnish examples of transverse bundles of storage cells, while those of 
Mertensia illustrate the more frequent arrangement in which the water tissue 
forms horizontal layers. 
2. The succulent form. Many succulent leaves are normal in shape and: 
size, though always thicker than ordinary leaves. Usually, however, they 
are reduced in size and are more or less cylindrical in form. The necessary 
decrease in transpiration is effected by the reduction in surface, the general 
stotage of water, a waxy coating, and, often also, by a very thick cuticle. 
Agave, Mesembryanthemum, Sedum, and Senecio furnish excellent ex- 
amples of this type. 
3. The dissected form. The reduction in surface is brought about by 
the division of the leaf blade into narrow linear or thread-like lobes which 
are widely separated. The 
latter are themselves protected 
by. a hairy covering or a thick 
cuticle, which is often sup- 
plemented by many rows of 
palisade, or by storage tissue. 
Artemisia, Senecio, and Gilia 
contain species which serve as 
good examples of this type. 
4. The grass form, Xero- 
phytic grasses and sedges have 
narrow filamentous leaves 
Fig. 34. Diplophyll of Mertensia linearis, with longitudinal furrows 
eae water cells (chresard, 3-94, light, 1). which serve to protect the 
stomata. The furrows are 
sometimes filled with hairs which are an additional protection, and the 
leaves often protect themselves further by rolling up into a thread-like 
shape. The elongated subulate leaves of Juncus and certain Cyperaceae 
are essentially of this type, although they are usually not furrowed. 
5. The needle form. ‘This is the typical leaf of conifers, in which a 
sweeping reduction of the leaf surface is an absolute necessity. The rela- 
tively small water loss of the needle leaf is still further decreased by a 
thick cuticle, and usualiy also by hypodermal layers of sclerenchyma. 
6. The roll form. Roll leaves are frequently small and linear, Their 
characteristic feature is produced by the rolling in of the margin on the 
under side, by which an almost completely closed chamber is formed for 
the protection of the stomata which are regularly confined to the lower 
surface of the leaf. The upper epidermis is heavily cutinized and the lower 
