LEAVES AND THEIR USES 245 



divided leaf of the same bulk. Plants living in dry localities 

 also are likely to have especially thick leaves. The century 

 plant, the bases of whose leaves are very thick, is an exam- 

 ple. A thick leaf not only exposes less surface in proportion 

 to its bulk and so reduces the danger of evaporation ; its 

 thick tissues also serve for the storage of water. The 

 juices of the thick leaves of some relatives of the century 

 plant are extracted by the Mexicans in great quantities and 

 are used in preparing the drink called " pulque." An ex- 

 treme reduction in size of leaves is seen in the various cac- 

 tuses, in nearly all of which the leaves are merely small 

 spines, and all the green, food-making tissue is contained in 

 the stem and branches. The branches of the prickly-pear 

 cactus not only do the work of leaves ; they actually look 

 like broad, thick leaves. 



In the case of plants of temperate regions with evergreen 

 leaves which remain alive for more than one year, the leaves 

 have much the same characteristics as have those of the 

 plants of dry regions. This is because much of the water in 

 the soil is frozen during the cold months, and so the water 

 supply of the roots is greatly reduced. Under such condi- 

 tions, if evaporation were to go on freely from the leaves, 

 they, and the plant as well, would die. This explains the 

 advantage of the thick cutin layer and the waxy coating of 

 such leaves as those of the evergreen hollies, the box, and 

 the European mistletoe. It explains, too, the advantage of 

 the small, needle-shaped leaves, well provided with cutin 

 and in some cases with wax, of the pines and spruces. These 

 latter are inhabitants for the most part of regions where the 

 winters are long and cold, and where there is great need to 

 guard against too rapid evaporation. 



259. Manufacture of Food in Leaves. Chlorophyl- 

 containing cells differ from all the other cells of living plants 

 and animals in their ability to manufacture carbohydrates 

 out of water and carbon dioxid. In most seed plants the 



