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 storage of water, a waxy coating, and, often also, by a very thick cuticle. Agave, Mesembryanthemum, Sedum, and Senecio furnish excellent examples of this type.
Fig. 34. Diplophyll of Mertensia linearis, showing water cells (chresard, 3–9%, light, 1). × 130.
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 supplemented 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. Xerophytic grasses and sedges have narrow filamentous leaves with longitudinal furrows 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 relatively small water loss of the needle leaf is still further decreased by a thick cuticle, and usually 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 one often protected by hairs. This type is found especially among the genera of the Ericales, but it also occurs in a large number of related families.
7. The scale form. Reduction of leaf surface for preventing excessive water loss reaches its logical culmination in the scale leaf characteristic of many trees and shrubs, e. g., Cupressus, Tamarix, etc. Scale leaves are leathery in texture, short and broad, and closely appressed to the stem, as well as often overlapping.
169. Types of stem xerophytes. In these types the leaves are deciduous early in the growing period, reduced to functionless scales, or entirely absent. The functions of the leaf have been assumed by the stem, which exhibits many of the structural adaptations of the former. Warming[[15]] has distinguished the following groups:
1. The phyllode form. The petiole is broadened and takes the place of the leaf blade which is lacking. In other cases, the stem is flattened or winged, and it replaces the entire leaf. This type occurs in Acacia, Baccharis, Genista, etc.