HYDROHARMOSE 1 19 



position during- midday that the rays strike them as obHquely as possible. A 

 leaf at right angles to the noonday sun receives ten times as much light 

 and heat upon a square decimeter of surface as does one placed at an angle 

 of 10 degrees. This device for reducing the intensity of insolation is best 

 developed in the erect or hanging leaves of many tropical trees. In tem- 

 perate zones, it is found in such plants as Silphium laciniatmn and Lactuca 

 scariola, and in species with equitant leaves. In such plants as Helianthus 

 anniius, the effect is just the opposite, since the turning of the crown keeps 

 the leaves for a long time at a high angle to the incident rays. In the case 

 of m.ats, it is the aggregation of plants which brings about the mutual pro- 

 tection of the leaves from insolation and wind. 



2. Rolling of the leaf. Many grasses and ericaceous plants possess leaves 

 capable of rolling or folding themselves together when drouth threatens. 

 In other cases, the leaves are permanently rolled or folded. The advantage 

 of this device arises not only from the reduction of surface, but also from 

 the fact that the stomata come to lie in a chamber more or less completely 

 closed. In the case of those mosses whose leaves roll or twist, a reduction 

 of surface alone is effected. 



3. Reduction of leaf. The transpiring surface of a plant is reduced by 

 decreasing the number of leaves, by reducing the size of each leaf, or by a 

 change in its form. In so far as the stem is a leaf, a decrease in size or a 

 change in shape brings about the same result. The finak outcome of reduc- 

 tion in size or number is the complete loss of leaves, and more rarely, of 

 the stem. Such marked decrease of leaf area is found only in intense 

 xerophytes, though it occurs in all deciduous trees as a temporary adapta- 

 tion. Changes in leaf form are nearly always accompanied by a decrease 

 in size. Of the forms which result, the scale, the linear or cylindrical leaf, 

 and the succulent leaf are the most common. Leaves which show a 

 tendency to divide often increase the number of lobes or make them smaller. 



4. Epidermal modifications. Excretions of wax and lime by the epidermis 

 have a pronounced effect by increasing the impermeability of the cuticle, 

 and, hence, decreasing epidermal transpiration. It seems improbable that 

 a coating of wax on the lower surface of a diphotic leaf can have this 

 ])urpose. The thickening of the outer wall of epidermal cells to form a 

 cuticle is the most perfect of all contrivances for decreasing permeability 

 and reducing transpiration. In many desert plants, the greatly thickened 

 cuticle effectually prevents epidermal transpiration. In these also the 

 cuticle is regularly developed in such a way as to protect the guard cells, 

 and even to close the opening partially. An epidermis consisting of two or 

 more layers of cells is an effective, though less frequent device against water 

 loss. When combined with a cuticle, as is usually the case, the imperme- 

 ability is almost complete. Hairs decrease transpiration by screening the 



