162 PLANT PHYSIOLOGY AND ECOLOGY 



5. The roll form. Roll leaves are frequently small and linear. 

 Their characteristic form is produced by the rolling in of the 

 margin on the under side. This forms an almost completely closed 

 chamber for the protection of the stomata, which are regularly 

 confined to the lower surface of the leaf. The upper epidermis 

 has a thick cuticle, and the lower one is often covered with hairs. 

 The roll type is found especially among the genera of the Ericales, 

 but it also occurs in a number of other families. 



6. The scale form. The reduction of leaves to scales represents 

 the extreme modification of the leaf under xerophytic conditions. 

 The next step results in the loss of the leaf and the assumption 

 of its functions by the stem. Scale leaves are short and broad, 

 leathery in texture, and closely appressed to the stem, as well as 

 often overlapping. They are characteristic of many trees and 

 shrubs, e.g., Cupressus, Tamarix, Thuja, etc. 



Experiment 49. Study of xerophytic leaves. Species representing the 

 above types of leaf xerophytes should be grown in the greenhouse in 

 so far as possible. Agave, Sedum, and Bryophyllum serve well for the 

 succulent leaf, Artemisia and Gilia for the dissected form, Sporobolus, 

 MuJilenbergia, Stipa, and Juncus for the grass form. Erica and Calluna 

 for the roll type, while conifers with needle or scale leaves are readily 

 found out-of-doors. Sketch a representative leaf of each type, and 

 estimate the total surface in square centimeters. Make cross-sections 

 and draw a segment from each leaf. 



181. Stem xerophytes. These are characterized by the absence 

 of leaves. In some plants the leaves are present at first, but fall early 

 in the season. In many cases the leaves are reduced to functionless 

 scales or are entirely absent. The functions of the leaf are trans- 

 ferred to the stem, which assumes many of the structural modifi- 

 cations of the former. The stem or some part of it often becomes 

 so changed that it is readily mistaken for a leaf. The following 

 kinds of stem xerophytes have been recognized, although not all 

 plants of these types are now xerophytes: (1) the phyllode form, 

 (2) the virgate form, (3) the rush form, (4) the cladophyll form, 

 (5) the flattened form, (6) the thorn form, (7) the succulent form. 



182. Types of stem xerophytes. 1. The phyllode form. The 

 petiole is broadened into a leaf-like structure or phyllode. It 

 replaces the leaf blade which is entirely lacking. In other cases 

 the stem is flattened or winged and takes the place of the whole 

 leaf. This type occurs in Acacia, Baccharis, Genista, etc. 



