THE LEAF . 89 



by a secretion from minute glands of the inner surface, 

 and is digested. 



155. The Bladderwort, one of the most interesting of our car- 

 nivorous plants, should be sought in still water of ponds and large 

 pools where it is common and examined under the lens. Nepenthes, 

 the East Indian Pitcher Plant, is not uncommon in greenhouses. In 

 nature it grows as an epiphyte on trees. 



156. The development of devices for entrapping animals, on the 

 part of the carnivorous plants, has the following significance. These 

 plants are found in places where nitrogenous compounds are scarce. 

 If their roots reach soil, it is merely wet sand or mud, poor in com- 

 bined nitrogen. Often the plants are aquatic or epiphytic. The 

 animals caught are rich in nitrogenous food, and so supply just that 

 nutritive element which could not otherwise be obtained. 



157. Duration of leaves. The leaves of such trees as the Elm, 

 Maple, Chestnut, Linden, and so on, last but a single season and then 

 fall off. Their leaves are deciduous; and the trees themselves are 

 spoken of as deciduous trees, meaning trees with deciduous foliage. 

 Evergreen leaves last more than one season at least. Those of the 

 Pines and Firs persist for two to five years, or in some cases more. 

 In the Conifer, Abies Pinsapo, the age of the leaf reaches sixteen or 

 seventeen years. 



158. The fall of deciduous leaves is not caused by their death. 

 Even before they begin to turn yellow in the autumn, the disarticulation 

 is begun which, when complete, allows them to drop away, leaving a 

 clean scar. Before this event, a large part of the useful substances in 

 the active tissue of the blade is withdrawn and saved to the plant. 

 The brilliant colors of autumn foliage are the signs that the living 

 matter is being chemically changed preparatory to this withdrawal. 

 Frost and cold have only an indirect effect, if any, in bringing about 

 the high coloration. 



The Arrangement of Leaves 



159. It has come to the student's notice in the study of buds and 

 of the stem that leaves are given off from the stem in somewhat defi- 

 nite fashion ; at least in such cases as that of the Horse-chestnut, 

 where they occur in pairs, on opposite sides of the stem. The regu- 

 larity would not be so apparent in the leafy branch of the Apple. 

 Yet here, too, a little attention shows a pretty definite system in the 

 disposition of the leaves. The study of leaf arrangement is called 

 Phyllotaxy. 



160. The attachment of the leaf to the stem is the insertion. Leaves 

 are inserted in three different modes. They are 



