I02 



General Botany 



way. Consequently the leaflets fall first and the petioles later. 



In the king-nut hickory and occasionally in the shagbark also, 



the petiole remains attached to the 

 tree through the following year 



(Fig. 53)- 



Self-pruning. A large number of 

 our common trees, like the cotton- 

 wood, maple, and elm, develop ab- 

 scission layers which cut off twigs 

 and sometimes branches an inch in 

 thickness. In these trees we have 

 twig fall as well as leaf fall. The 

 faUing of flowers, and of fruits hke 

 apples and nuts, is also due to the 

 softening or dissolving of abscission 

 layers formed across the stem at the 

 point of attachment. Sometimes 

 abscission is of advantage to the 

 plant, sometimes it is quite disad- 

 vantageous. 

 Evergreen and deciduous trees. In the Northern states many 

 persons have come to think that the evergreen habit is associated 

 only with needle leaves, because in the North the evergreens are 

 mostly of the needle-leafed type. But in the Southern states 

 there are many broad-leafed trees, hke the magnoha, rhododen- 

 dron, and holly, that are also evergreen. Moreover, the tama- 

 racks of the North and the bald cypress of the South furnish 

 examples of needle-leafed trees that are deciduous. If we include 

 the shrubs, there are many broad-leafed plants, both in the North 

 and in the South, that have the evergreen habit. In the tropics 

 most of the trees are evergreen, and almost all have broad leaves. 

 It must be noted that even in the case of evergreens individual 

 leaves remain on the trees for only a hmited number of years. 

 The leaves of the evergreens of temperate regions are quite dif- 



FiG. 54. Abscission of branches of 

 the Cottonwood. Twigs and small 

 branches, as well as leaves and fruits, 

 are cut off by the formation of abscis- 

 sion layers. 



