=162. Modifications due to a small water supply.= A water supply which may become deficient at any time is compensated either by changes which decrease transpiration, or by those that increase the amount of water absorbed or stored. These operate upon the form and size of the organs concerned, as well as upon their structure. Modifications of the form of leaf and stem are alike in that they lessen transpiration by a reduction of the amount of surface exposed to the air. Structural adaptations, on the other hand, bring about the protection of epidermal cells and stomata, and often internal cells also, from the factors which cause transpiration, or they anticipate periods of excessive transpiration by the storage of water in specialized cells or tissues. In certain extreme types the epidermis is itself modified for the absorption of water vapor from the air. =163. The decrease of water loss.= The following is a summary of the contrivances for reducing transpiration. 1. _Position of the leaf._ Since the energy of a ray of sunlight is greatest at the sun’s highest altitudes, those leaves transpire least which are in such a 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 temperate zones, it is found in such plants as _Silphium 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 protection 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.