302 



PEACTICAL BOTANY 



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. 



Leaves of pines are not literally evergreen, as is sometimes 

 supposed. In different pines the leaves remain on the branches 

 different lengths of time. In all species, after a period ranging 

 I __ _ from two to four years the 



older leaves fall. There 

 is no definite brief period 

 when all the leaves are 

 discarded, as in deciduous 

 plants, but they fall a few 

 at a time. 



The leaf bases are spi- 

 rally arranged upon the 

 branches. This may be ob- 

 served either on the leafy 

 branch or by means of the 

 leaf scars left upon branches 

 from which the leaves have 

 fallen. 



279. The structure of 

 needle leaves. The hard sur- 

 face of the leaves is due 

 to the very heavy-walled 

 epidermis and several un- 

 derlymg layers of heavy- 

 walled strengthening cells 

 (sclerenchyma) (Fig. 249). Stomata in the epidermis are 

 deeply placed, and oftentimes their pores are clogged with 

 dust so that they appear quite dark. Beneath the strength- 

 enmg tissue is the chlorophyll tissue, through which run the 

 resin ducts. In the interior of the leaf is the pith region, 

 through which run two groups of fibrovascular bundle cells. 

 The well-protected chlorophyll tissue seems able to with- 

 stand severe conditions. Its temperature changes probably 



Fig. 248. A branch of a pine 



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

 the tip ol the shoot (s) a very young cone 

 (j/c) just open and ready to receive pollen. 

 On the young shoot are the young needle 

 leaves, and at the tip is the hud (6), which 

 continues the growth of the stem 



