. Research methods in ecology. Plant ecology. HYDROHARMOSE 123 intensely xerophytic habitats, and, consequently, it is difficult, if not unde- sirable, to place such xerophytes under a particular group. The best that can be done is to recognize the types arising from extreme or characteristic modification, and to connect the less marked forms as closely as possible with these. Halophytes differ from xerophytes only in the fact that the chresard is determined by the salt-content of the habitat, and not by the tex- ture of the soil. In consequence, they should not be treated as a distinct group.
. Research methods in ecology. Plant ecology. HYDROHARMOSE 123 intensely xerophytic habitats, and, consequently, it is difficult, if not unde- sirable, to place such xerophytes under a particular group. The best that can be done is to recognize the types arising from extreme or characteristic modification, and to connect the less marked forms as closely as possible with these. Halophytes differ from xerophytes only in the fact that the chresard is determined by the salt-content of the habitat, and not by the tex- ture of the soil. In consequence, they should not be treated as a distinct group. 168. Types of leaf xerophytes. In these, adaptation has acted primarily upon the leaf, while the stem has remained normal for the most part. Even when the leaves have become scale-like, they persist throughout the grow- ing season, and continue to play the primary part in photosyn- thesis. The following types may be distinguished: I. The normal form. The leaf is of the usual dorsiventral character. In place of a reduc- tion in size, structural modifica- tions are used to decrease transpiration. With respect to the protective feature that is predominant, three subtypes may be recognized. The cutinized leaf compensates for a low water-con- tent by means of a thick cuticle, often reinforced by a high de- of palisade tissue. Such leaves are more or less leathery, and they are often evergreen also. Arctostaphylus and many species of Pentstemon are good examples. Lanate leaves, i. e., those with dense hairy coverings on one or both surfaces, as Artemisia, Antennaria, etc., regularly lack both cuticle and 'palisade tissue. The protection against water loss, however, is so perfect that the chlorenchym often assumes the loose structure of a shade leaf. Storage leaves usually have a well- developed cuticle and several rows of palisade cells, but their characteristic feature is the water-storage tissue, which maintains a reserve supply of. Fig. 33. Staurophyll of Bahia disseda
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