DIV. I 



MORPHOLOGY 



155 



interruption of growth, annual rings may also be wanting, but in many cases zones 

 resembling the annual rings occur. 



The water-conducting elements of the most recently formed annual rings are 

 the only ones that are in direct connection with the leaves of the corresponding 

 period of vegetation. Since there is a sudden demand for a considerable amount 

 of water for transpiration when the leaves unfold in the spring, the provision of 

 conducting channels in the spring wood is readily comprehensible. In many 

 woody plants the foliage is not further increased during the summer, and the 

 cambium can therefore form mechanical tissue in the autumn wood. 



FIG. 180. Portion of a transverse section of the wood of Tilia -ulmifolia. m, Large pitted 

 vessel ; t, tracheides ; ?, wood-fibre ; p, wood parenchyma ; r, medullary ray. (x 540. After 

 STRASBURGEK.) 



In spite of the variety in the structure of the wood of Dicotyledons 

 there are some constant features in the arrangement of the different 

 tissues. The vascular strands composed of tracheae and tracheides, 

 while they ramify in the radial and tangential directions, form 

 continuous longitudinal tracts from the roots to the finest tips of the 

 branches. Were this not so the needs of the shoot-system as regards 

 its water supply would not be met. Wood parenchyma (Figs. 180, 

 181 p), which is well developed in most dicotyledonous woods, also 

 forms longitudinal strands or layers which, however, end blindly 

 above and below. These form along with the medullary rays a 

 connected system of living cells. The vessels always stand in 

 connection with these living cells, being sometimes surrounded 



