SUBKINGDOM SPERM ATOPHYTA 



335 



In most Conifers there are regular periods of growth, followed by 

 a dormant period, which in northern regions falls in the winter. 

 With the sudden renewal of activity in the spring, the growth of 

 the young tracheids is especially marked, and these first formed 

 tracheids are much larger in the radial diameter, and have thinner 

 walls than the tracheids last formed in the autumn. This results in 

 the sharp line between the rings of wood marking two successive 

 years' growth. Under normal conditions, one growth-ring is formed 

 each year, and the rings of wood constitute a very fair index of 

 the age of the tree. It is probable that the largest of the living 

 Sequoias are two thousand to twenty-five hundred years old. 



The Leaf 



The leaves of the Conifers may be inserted singly upon the shoot, 

 as in Taxus and Tsuga (Figs. 308, 309) ; or they may be in clusters, 

 or fascicles upon special 

 short shoots, as in Pinus, 

 Cedrus, and Larix. In the 

 latter, and in the Bald- 

 cypress (Taxodium) of the 

 Gulf states, the leaves are 

 shed each year. In most 

 Conifers they persist for 

 several years. 



s- 



Each leaf receives a single 

 vascular bundle from the stem. 

 This may remain undivided, or 

 it may divide into two or more. 

 A transverse section of the leaf 

 of Pinus (Fig. 299) shows the 

 epidermal cells to be very thick- 

 walled, and the stomata are 

 sunk in pits, overlying an air- 

 space in the mesophyll. Below Fl - 298. A, Pinus Coulteri, branch showing 

 the epidermis is a greater or less scars, s, where the short leafy shoots (fascicles), 

 . , , , ... I. have fallen off (natural size). B, Cedrus 



amount of hypodermal tissue, 7w - 6an ^ showing m a ny .leaved fascicle and two 

 composed of colorless, very single leaves, Z (natural size) . 

 thick-walled fibrous cells, with 

 connecting canaliculi between their cavities. 



The green tissue is composed of irregular cells, with large intercellular spaces. 

 In Pinus, these cells have conspicuous infoldings of the cell-walls, which in- 

 crease the area occupied by the chloroplasts. 



In the middle of the leaf are the two vascular bundles, closely resembling 

 those of the stem. The xylem is turned toward the upper side of the leaf. Sur- 

 rounding the two bundles is a large oval area (in section), sharply separated, by 

 a definite row of cells, from the green mesophyll. 



