. Méthodes de recherche en écologie. L'écologie de la plante. 124 L'usine de l'eau pour le moment d'extrême drouth. Espèces d'Helianthus xérophytiques exarnples fournit des ensembles de cellules de stockage transversal, tandis que ceux de Mertensia illustrent l'arrangement plus fréquente dans laquelle le tissu de l'eau forme des couches horizontales. 2. Le formulaire de succulentes. De nombreuses feuilles succulentes sont de forme normale et la taille, mais aussi toujours plus épais que les feuilles ordinaires. Habituellement, cependant, ils sont de taille réduite et sont plus ou moins de forme cylindrique. La nécessaire diminution de la transpiration est effectuée par la réduction de surface, le gener

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. Research methods in ecology. Plant ecology. 124 THE PLANT water for the time of extreme drouth. Xerophytic species of Helianthus furnish exarnples 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 storage of water, a waxy coating, and, often also, by a very thick cuticle. Agave, Mesembryanthemum, Sednm, 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 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 /uncus 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 usually also by hypodermal layers of sclerenchyma. 6. The roll form-. Roll leaves are frequently small and linear. Their characteristic featu

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