DIV. I 



MORPHOLOGY 



157 



In Dicotyledons also, as is very well shown in the Willow, the marginal cells 

 of the medullary rays usually stand in relation to the adjacent water-conducting 

 elements by means of pits bordered on one side ; these living cells are higher 

 than those of the middle rows (Fig. 182 tm}. The latter are more extended 

 radially and have no special connection with the water-conducting elements. 

 They serve for the conduction and storage of'assimilated materials (Fig. 182 sra). 



The parenchymatous cells of the medullary rays and of the wood which 

 adjoin the vessels take water from the latter and hand it on to the other living 

 cells. In spring, on the other hand, they pass a large part of the stored assimilated 

 material (especially glucose and small amounts of albuminous substances) into the 

 vessels, so that these substances can be quickly transported to the places where 



FIG. 182. A radial section of the wood of 

 Tilia ulmi folia, showing a small medul- 

 lary ray. g, Vessel ; I, wood fibres ; tm, 

 medullary ray cells in communication 

 with the water-channels by means of 

 pits ; sm, conducting cells of the 

 medullary ray. (x 240. After 

 SCHEXCK.) 



FIG. 183. Transverse section of a vessel from 

 the heart-wood of Robinia Pseudacacia, closed 

 by thy loses ; at a, a is shown the connection 

 between the thyloses and the cells from which 

 they have been formed. (x 300. After 



SCHENCK.) 



they are required. Owing to this, sugar and proteids can be demonstrated in the 

 vessels during the winter and early spring. These substances are also present in 

 the sap that exudes when holes are bored in the stems of Birch, Maple, and other 

 trees in the spring. 



Grain of the Wood. The technical value of certain woods is affected not 

 only by the colour but by the graining. This depends in the first place on the 

 arrangement of the annual rings and medullary rays, but also in many cases 

 (e.g. Hazel) upon a wavy course of the elements of the wood ; this may be brought 

 about by the crowded arrangement of lateral or adventitious buds or lateral roots, 

 or by the stimulus of wounding. 



C. Subsequent Alterations of the Wood. In the majority 

 of trees the living elements in the more centrally - placed older 

 portions of the woody mass die and the water channels become 

 stopped up, leading to the formation of what is known as the 



