284 



INTEODUCTION TO BOTAKY 



needle leaves grow. The needle leaves are really continua- 

 tions of these small branches. The inward faces of the leaves 

 are so arranged that all of one cluster, when put together, 

 compose a cylindrical leaf mass. That is, when two leaves 

 compose the cluster, the leaf branch is divided into halves; 

 when three or five are in one cluster, the branch is divided 



into three or five parts. 



Gymnosperms are chiefly 

 evergreen (that is, keep their 

 old leaves until after new 

 ones have come), but some 

 of them, as the larch, or 

 tamarack, and the bald cy- 

 press, are deciduous (that 

 is, shed their old leaves 

 before the appearance of 

 the new ones). The periods 

 during which the leaves 

 endure, range in different 

 species from two to four 

 years. By determining the 

 age of the branches through 



FIG. 220. A branch of a pine a study of the yearly bud 



At the left is a one-year-old cone (c) , and at scars one may readily ascer- 



the tip of the shoot () a very young cone tain how long the leaves 



(yc) just open and ready to receive pollen. 1 . 



On the young shoot are the young needle last On any pine tree, 

 leaves, and at the tip is the bud (6), which The clusters of pine leaves 

 continues the growth of the stem , . ,, , 



are arranged spirally around 



the stem, as may be learned, when they have fallen, by an 

 examination of the leaf scars. 



269. Internal structure of needle leaves. The stiffness of 

 pine leaves is one of their most noticeable features, and when 

 we examine a cross section, we are able to locate the tissues 

 that give the leaves their rigidity. The outer layer of cells, 

 the epidermis (fig. 221), has an extremely heavy covering, the 

 cuticle; beneath the epidermis there are other heavy- walled 



