ADAPTATION TO WATER 159 



quent arrangement in which the water tissue forms horizontal 

 layers, while certain species of Helianthus, Grindelia, Psorolea, etc., 

 have this tissue disposed in transverse bundles or rows. 



179. Lanate leaves are those with dense hairy coverings upon 

 one or both surfaces. The form of the hairs varies widely in 

 different species, from short, often glandular ones to those that 

 are repeatedly branched or curved in various ways. An epidermis 

 covered with a dense layer of hairs regularly lacks a cuticle. ]\lore- 

 over, the protection against water loss is so perfect that the chloren- 

 chym often assumes the loose structure found in shade leaves. A 

 very large number of plants, e.g., Antennaria, Artemisia, Tetra- 

 neuris, etc., obtain their protection against drouth by means of 

 hairs. In a few cases, the latter are confined to the upper surface, 

 but as a rule they are nearly or quite as abundant upon the lower 

 surface also. 



Experiment 48. Studj'^ of normal leaf xerophytes. Cut cross- 

 sections, preferably by means of the microtome, of the three types of 

 the normal leaf. ]\Iake a careful drawing of a segment across each, 

 and in addition outline the entire leaf. Compare the three types crit- 

 ically, especially with respect to the various protective devices. This 

 comparison is most striking when the three species concerned can be 

 found in the same habitat. 



180. Other leaf xerophytes. Species that have lost the normal 

 form of the leaf in response to dryness have often found it also 

 necessary to employ additional protection. Consequently, they 

 may show a thick cuticle, a hairy covering, or storage tissue. In 

 all of the following types, reduction of the leaf surface is the charac- 

 teristic feature, though this result may be arrived at in ^'arious 

 ways, e.g., by thickening the leaf, by lobing, by rolling, etc. 



1. The succulent form. Many succulent leaves are normal in 

 shape and size, though they are always thicker than ordinary 

 leaves. Usually, however, they are reduced in size, and more or 

 less cylindrical in form. The necessary decrease in transjiiration 

 is secured by reducing the surface, and by storing water uniformly 

 throughout the leaf. The latter is usually covered A\-ith a waxy 

 coating, and often possesses a very thick cuticle. The cliaracter 

 of the leaf arises from its unusual ability to store water, which 

 forms the chief protection of the plant. The stored water is licld 

 very firmly in opposition to the ])ull of evaporation. This ])rojierty 



