14 



ELEMENTS OF BOTANY. 



egress of liquids by osmose. Later their walls become thickened by the 

 deposition of cellulose, etc., and lose their transparency. The wood-cells 

 of some plants are of a characteristic form, which may serve, as in the 

 coniferse, for the identification of the order. 



But insiDection of a cross-section of almost any exogenous stem will 

 show that the woody tissue is not uniformly solid throughout, but is trav- 

 €i-sed by many small canals (Fig. 23). In some stems, as that of the grape- 

 Tine, these are so large that one can readily draw water through them by 

 suction Avith the lips. These canals are called ducts or vessels, and are 

 formed from large cells i^laced end to end, the cell- walls at their point of 

 contact afterward beino' absoi'bed. 



Fig. 23. — Horizontal and vertical section of the stem of a maple. Magnified. 



Nor is this all. A longitudinal section of many stems shows glistening 

 plates of tissue traversing the wood from tlie pith toward the bark ; these 

 plates, called medullary rays, are formed, like the pith, of cellular tis- 

 sue, and serve as a means of communication between the jDith and the ex- 

 ternal growing surface of the stem (Fig. 24). The medullary rays of oak 

 and sugar-maple are highly developed, forming the so-called satin grain of 

 the wood. 



Inspection of a cross-section of any exogenous stem of a few years' 

 growth will show that the wood is made up of concentric rings 

 (Fig. 19). These rings represent annual accessions to the wood previously 

 iormed, each one comprising the growth of a year. Each is complete in 

 itself, and, though more or less strongly adherent to the one which it encir- 

 cles, evidently is, at the time of its growth, the only growing part of the stem. 

 Once formed, these rings afterward undergo but slight changes in char- 

 acter, and never any in volume. As years pass by they become more dense, 

 and generally more or less deeply colored by the deposition of coloring 

 anatters. Hence, after a few 3'^ears' growth most exogenous stems present 



