14 



ELEMENTS OF BOTANY. 



egress of liquids hy osiuose. Later tlieir walls become thickened by the 

 deijositiou uf celliiloso, etc., iind lose their transparency. The wood-cells 

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

 conifene, for the identification of the order. 



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

 show tha< the woody tissue is not unif(jrnily solid throughout, Init is trav- 

 ersed by many small canals (Fig. 215). In souie stems, as that of the grape- 

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

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

 formed from large cells placed end to end, the cell- walls at their point of 

 contact afterward being absorbed. 



!i mtini,.:;'iii:-t-aa.if;,jji.yAA 



Fio. 23. — Horizontal and vertical section of the stem of a maplo. Magnified. 



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

 l^lates of tissue traversing the wood from the 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 pith 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 ci-oss-soction of any exogenous stem of a few years' 

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

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

 formed, each one comprising the gi'owth of a year. Each is complete in 

 itself, and, though more or less sti'ongly adherent to the one which it encir- 

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

 Once formed, these rings afterward undei'go 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 

 matters. Hence, after a few years' growth most exogenous stems present 



