382 RELATION TO ENVIRONMENT. 



is composed ordinarily of angular, little elongated, parenchyma cells, 

 when mature mostly without cell-contents and filled with air. The pith 

 region in different trees is quite diversified. It may be hollow, chambered, 

 contain scattered thick-walled cells, have woody partitions, or rarely be 

 entirely thick- walled. 



The nature of the woody ring is rather perplexing at first; but its origin 

 is simple. We may conceive that it has developed from a stem-type like the 

 sunflower, in which the bundles, though separate, are connected by a con- 

 tinuous cambium ring. In the woody twigs the numerous bundles are 

 closely packed together, and only separated by the primary pith-rays ex- 

 tending from the pith to the cortex. Other secondary pith-rays are pro- 

 duced within each bundle, but they usually extend only part way from 

 the cortex to the pith. The wood represents the xylem of the bundle, 

 and the sieve-tubes of the bark, the phloem. 



744. Growth in thickness. Although the year's growth does not in- 

 crease in length after the first season has passed, it does increase in dianv 

 eter very much. From the size of an ordinary little twig it may at length 

 become a large tree trunk several feet in thickness. Only a portion of the 

 first year's growth is produced by the growing point. All the rest is a 

 product of the cambium, a cylinder of wood being added to the exterioi 

 of the old wood each season. The cambium, here, as in the sunflower, lies 

 between the phloem and the xylem, forming a cylinder entirely around 

 the stem. In spring, when active, it becomes soft and delicate, thus en 

 abling one to easily strip off the bark from some trees, such as willow, etc., 

 at that season. 



745. Annual rings in woody stems. The wood produced by the cam. 

 bium each season is not homogeneous throughout, but is usually much 

 denser toward the outer part of the yearly cylinder, wood-fibers here pre- 

 dominating. In the inner portion vessels predominate, giving a much 

 more porous effect. The transition from one year's growth to another 

 is very abrupt, giving rise to the appearance of rings in cross- section. Since 

 ordinarily in temperate climates but one cylinder of wood is added each 

 year, the number of rings will indicate the age of the trunk or branch. 

 This is not absolutely accurate, since in some trees under certain conditions 

 more than one ring may be produced in a summer. The porous part 

 of the ring is often termed "spring wood," and the denser portion "fall 

 wood," but since growth from the cambium ceases in most trees by the 

 middle of July, "summer wood" would be more appropriate for the latter. 

 It is mainly the alternation of the cylinders of the spring and summei 

 wood that gives the characteristic grain to lumber. Pith-rays play an 

 important part in wood graining only in a few woods, as, for instance, in 

 quartered oak. The reason for the production of porous spring wood 

 and dense summer wood is still one of the unsolved problems of botany. 



