LEAVES AND THEIR WORK 



129 



Another very important use to man is seen in the fact that leaves, 

 falling to the ground, help to form a rich covering of humus, which 

 acts as a coat to hold in moisture. The forests are our greatest 

 source of water supply. The cutting away of the forest always 

 means a depletion of the reserve water stored in soil, with conse- 

 quent floods and droughts in alternation. 



Leaves are used directly by man for food. Examples are cab- 

 bage, lettuce, kale, broccoli, and some others. These foods, 

 properly admixed with certain fleshy foods, are of great importance 

 in giving a balance to diet. In a wider sense, all animals depend 

 upon leaves for their food supply 

 either directly, for herbivorous ani- 

 mals feed upon the leaves of plants 

 or indirectly in foods obtained from 

 roots, stems, seeds, and fruits. For 

 in every case the stored food has been 

 manufactured in the leafy part of the 

 plant and transported within the plant 

 to its place of storage. Even meat- 

 eating animals are in the long run 

 dependent upon plants, for they feed 

 upon plant eaters. 



Modified Leaves. In many plants the 

 leaves are reduced to spines or have part 

 of the leaf modified so as to form spines. 

 In some leaves this appears to be for pro- 

 tection against animals, but in some cases, 

 as the cactus, it is a means of protecting 

 the plant against loss of water through 

 evaporation. 



If a cactus is cut open, it will be found 

 to contain a very considerable amount of 

 water. The Indians of the New Mexican 

 desert region, when far from a source of 

 water, sometimes cut off the top of a large 

 cactus, mash up the soft interior of the thickened stem, squeeze out the 

 pulp, and thus obtain several quarts of drinkable water. 



Protection by Hairs. In the mullein, one of our hardiest weeds, the 

 leaf is covered with a coating of finely branched hairs. Might such a 

 covering be of use to the leaf ? In what ways ? 

 HUNT. ES. BIO. 9 



A cactus, showing the leaves 

 modified into spines. 



