STEMS, KOOTS, AND LEAVES 103 



composed of three layers. The outer layer is the greatly thickened 

 cork layer, which is now seamed and cracked by the pressure exerted 

 by the expanding wood cylinder. The middle layer is the remnant 

 of the former green bark, or cortex, now composed of dead tissue 

 squeezed between the outer cork layer and the phloem. The outer 

 portions of the phloem form the inner layer of the dead bark, made 

 up of layers of thick-walled phloem fibers alternating with crushed 

 and dead sieve tubes and phloem parenchyma. This outer portion 

 of the phloem constitutes a kind of exoskeleton for the tree, 

 forming with the other layers of the bark an effective protection 

 against the loss of water and heat, as well as a means of warding 

 off mechanical injury. The inner phloem is replaced each year by a 

 new layer produced by the cambium. This new annual layer is the 

 effective portion of the phloem in conducting and storing food, since 

 the average life of the sieve tubes in most trees is limited to a single 

 season. The central portion of the wood cylinder has been converted 

 into dead heartwood, and the pith has disappeared. The sapwood 

 now forms the living, active portion of the wood, furnished with the 

 three main systems of tissues discussed in the preceding pages : 

 namely, the conducting water ducts ; the storage system of living 

 cells, made up of wood rays and xylem parenchyma ; and the mechan- 

 ical fibrous elements constituting the endoskeleton. 



Longevity of the tree. The tree, unlike any othef^known organ- 

 ism, is so constituted as to be able to perpetuate its life through an 

 almost indefinite period of time. A new crop of leaves is formed each 

 season for the making of food by photosynthesis, while the roots 

 invade a constantly increasing area of soil for the absorption of water 

 and solutes in the form of soil salts. The supporting trunk is enabled, 

 through its outer and inner cambium layers, to increase the thickness 

 and effectiveness of its protective bark and also of its inner skeleton of 

 xylem and phloem fibers. At the same time, as the central wood be- 

 comes transformed into dead mechanical elements, the cambium forms 

 new layers of conducting and storing tissues in the sapwood, which 

 keep pace with the increase of its absorbing and manufacturing sur- 

 faces. This power of constant growth, throughout a period extending 

 over hundreds of years in the case of the longest-lived trees, is 

 wholly due to the fact that certain cell groups, which we have called 

 meristems and cambiums, retain a condition of perpetual youth and 

 activity, which enables them to supplement dead and useless tis- 

 sues with an annual growth of new and active ones. The long period 



