CHARACTER OF LEAVES. 73 



The circumference of the leaf presents variously formed outlines, 

 tips and margins which will be recognized by the observer without any 

 other description than that found in the glossary. 



The insertion and direction of leaves in regard to the stem will also 

 be seen to be characterized by numerous terms, most of which are 

 burdensome, unless the subject is studied minutely. 



The size of leaves greatly differs. They increase in magnitude to- 

 wards the tropics, where some are of immense size. Those of the 

 Talipot-tree in Ceylon, it is said, will cover 20 men each. Numerous 

 ..and important uses are made of them, especially the palm leaves, in 

 warm and rainy latitudes. The size of the leaf, however, bears no 

 proportion to the tree or shrub, as seen in the oak and burdock. 



The color of leaves, though generally green, is beautifully varie- 

 gated in autumn, yet never so beautifully as the corolla of flowers. 

 Green is most agreeable to the eye ; and it is remarkable that leaves 

 are so constituted as to reflect that color, mostly. The changes in 

 this color are effected by the combination of oxygen with iron, as in 

 the colored petals of flowers. 



The duration of leaves is characterized as caducous when they fall 

 off before the end of summer ; diciduous when they fall at the begin- 

 ning of winter, as with most leaves in our climate, or 30 or 40 degs. 

 from the equator ; persistent when they remain on the tree or shrub 

 during the changes of season, as with the pine, box, etc., and evergreen 

 when they preserve their green color throughout the year, as with 

 pines and most resinous trees. These leaves are indeed annually 

 changed, but the young leaves appear before the old ones decay. 

 Leaves, in the torrid zone, are generally persistent and evergreen ; 

 but, if the plants be removed to a colder climate, they become annual 

 and lose their foliage annually, as with the passion flower, a native of 

 a warmer climate. 



The defoliation of plants is the falling off of the leaf. It is 

 attributed to the death of the leaf and the cessation of the vital 

 principle in the parts to which it is attached. On a tree struck 

 by lightning, which does not possess the power to throw off 

 the leaves, they adhere to the dead branches. The accomplish- 

 ment of the important functions of the plant, such as the formation 

 of its buds, flowers and fruit, and the exhaustion of its energies, 

 with the occurrence at that time of frosts and winds, are the general 

 causes of defoliation. The leaves of some trees turn red, others brown 

 and others yellow about the middle of autumn, when they present to the 

 eye, in an American forest, the richest scene imaginable. Some few 

 plants have no leaves, as the mushroom, indian pipe, etc., nor is it 

 known how the deficiency is amended. In the renewal of the leaf 

 a leaf-scale appears at the base and also a floral leaf in flowers. 



The. anatomy of leaves. The skeleton or frame of a leaf may be 

 7 



