We find, too, that the cacti with most pronounced thorny growth live in the hottest parts of the desert, where the thermometer often registers 130° Fahrenheit during the long summer days and sometimes up to 150°! Their dense layer of spines becomes a shield of lacework, protecting the plants by cutting off over twenty per cent of the light, and reducing the terrible heat by raising the humidity within the network of spines, which in turn reduces evaporation from the plant. If it were not for their thorns and sheaths cacti would be scalded by the burning temperatures of even one summer day in the great desert amphitheater of the sun. Being resinous, cactus thorns are very inflammable, and if ignited they all burn to a cinder before the fire ceases, for one cluster of spines will set others aflame and so the fire sweeps over the entire plant, rapidly changing the beautifully colored, symmetrical and translucent spines into ugly charred masses. The cactus is encased in a thick cuticle which is continuous over the whole surface of the plant, except at the numerous, small, rounded or oval areas of growth called areolas. In the cactus all growth, of leaves, spines, spicules, flowers, and even roots (in the case of cuttings) and branches, takes place from these areolas, which are truly areas or centers of growth; and if all these areolas are cut away on the stem and its tip cut off, the plant ceases to grow and dies. Spines are generally understood to represent branches, since they grow from the tissues of the plant under the epidermis and not from the epidermis. The inordinate multiplication of spines in cacti is not well understood by botanists. Some cacti add thorns to their spine clusters in the areolas each year, and thus in time the cluster may come to have as many as fifty or even a hundred spines on old stems and large branches. Occasionally cactus spines replace themselves in areolas after the former thorns have been destroyed or burned off. The thorns of some cacti may grow as long as six inches and even longer, and as broad as one-fourth inch at their bases. After they complete their growth toward the close of the season and the bases become hard and firm, they do not elongate farther nor make further growth.