DIV. I 



MORPHOLOGY 



153 



their walls. These tracheidal cells facilitate the conduction of water in the radial 

 direction between the tracheides, which are only pitted on their radial walls. In 

 most other Conifers, in which such tracheidal elements in the medullary rays are 

 wanting, there are tangentially-placed bordered pits in the tracheides of the wood, 

 and these allow of the movement of water in 

 a radial direction. The parenchymatous cells 

 of the medullary rays of the wood are connected 

 with the tracheides by means of large pits 

 bordered on one side (Fig. 177 et). 



Owing to climatic variations, the 

 cambial tissue of Gymnosperms, as of 

 most Dicotyledons, exhibits a periodical 

 activity which *is expressed by the for- 

 mation of ANNUAL RINGS ( 79 ) of growth 

 (Figs. 178, 179). In spring, when new 

 shoots are being formed, wider tracheal 

 elements are developed than in the follow- 

 ing seasons (Fig. 175^4). For this reason 

 a difference is perceptible between the 

 EARLY WOOD (spring wood), which is 

 composed of large elements especially 

 active in the conveyance of water (Fig. 

 175/), and the LATE WOOD (autumn 

 wood), consisting of narrow elements 

 which impart to a stem its necessary -;- > '^~< 



rigidity (Figs. 175 A, s, 179). Through- ' v c -\ 



out the greater part of the temperate L/ 



zone, the formation of wood ceases in 



the latter part Of AugUSt Until the follow- Fl - m - Tangential section of the 

 , , - autumn wood of a Pine, t, Bordered 



ing spring, when the larger elements of 



pit ; tm, tracheidal medullary ray cells; 

 sm, medullary ray cells containing 

 starch ; et, pit bordered only on one 

 side; i, intercellular space in the 

 medullary ray. (x , 240. After 

 SCHENCK.) 



the spring wood are again developed. 

 Owing to the contrast in the structure of 

 the spring and the autumn wood, the 

 limits (Figs. 175 g, 179 i) between suc- 

 cessive annual rings of growth become 

 so sharply denned as to be visible even to the naked eye, and thus 

 serve as a means of computing the age of a plant. The limits 

 between the annual rings are less evident in the root, all the wood 

 resembling spring wood. The cambium of the root may remain active 

 throughout the winter and only pass into a resting condition at the 

 commencement of the new vegetative period. 



In a stem or root that has undergone secondary thickening fewer annual rings 

 will be seen on the cross-section the nearer this is made to the growing point. 

 The older annual rings and the older layers of bast disappear in order of their age 

 as the tip is approached. 



Under certain conditions the number of annual rings may exceed the number of 

 years of growth. When the leaves are destroyed by frost, caterpillars, or other 



