HYDROHARMOSE IIg 
position during midday that the rays strike them as obliquely 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 Silphiuim laciniatum and Lactuca 
scariola, and in species with equitant leaves. In such plants as Helianthus 
annuus, 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 mats, 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 final 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 inteuse 
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 
purpose. 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 usuaily the case, the imperme- 
ability is almost complete. Hairs decrease transpiration by screening the 
