THE GYMXOSPERMAE : COXIFERALES AND TAXALES 667 



long and pointed at both ends. They are almost square in section, and their 

 radial walls are marked by a single row of large bordered pits. 



Simple pit 



Bordered pit 



Fig. 666. — Pinus sylvestris. Longitudinal radial section of 

 the secondary wood showing bordered pits on the 

 tracheids and small simple pits in the medullary ray. 

 The medullan,' ray is cut longitudinally and is seen 

 crossing the tracheids in a horizontal direction. 



The w^ood is not uniform throughout, for in transverse section it is possible 

 to recognize rings, termed annual rings, each of which represents a year's 

 growth (Fig. 667). The occurrence of these rings can be explained if we 

 realize that the size of the tracheids produced in the autumn, w^hen the supply 

 of food material is slowdng down, is smaller than that of those produced in 

 the spring, when with the first burst of activity large quantities of food 

 material are available and larger tracheids are laid down. Counting the 

 number of such rings gives an estimate of the age of the tree or branch. 



The walls of the autumn tracheids are much thicker than those in the 

 spring wood and their walls show a spiral striation of fine lines. The openings 

 of their bordered pits are not circular but narrowly elliptical and lie at right 

 angles on opposite sides of the wall, so that each pit, in surface view, seems 

 to be marked with a cross. The last row of tracheids in each annual ring is 



