8o 



THE ANATOMY OF WOODY PLANTS 



black on the periphery of the stem. Centrally placed is the 

 pith, surrounded by the woody cylinder, which in turn is circled 

 by the phloem and the cortex. The woody tissues are encroached 



upon by three deep 

 bays extending 

 frojn the medulla 

 and subtended by 

 the three leaves. 

 These extensions 

 from the pith mark 

 the presence of the 

 leaf gaps, interrup- 

 tions in the con- 

 tinui ty of the 

 woody cylinder re- 

 lated to the passing 

 out of the foliar 

 traces. Later the 

 intervals in the 



FIG. 60. Tangential section of the wood of Casua- 

 rina eqiiisetifolia. Explanation in the text. 



wood are covered 

 by the activity of 

 the cambium, so 



that the cylinder becomes continuous 

 in the second or third year of growth. 

 Clearly there is no structural feature 

 of importance in the woody cylinder 

 related to the leaf trace other than the 

 foliar gap. This is the general situa- 

 tion in the case of coniferous stems as 

 well as in that of their Paleozoic ances- 

 tors, the Cordaitales. 



Having diagrammatically compassed 



the organization of the stem in the coni- FlG ' 6i.-Diagrammatic 



transverse section of a coniferous 



fers, we are in a position to consider the twig . Explanation in the text, 

 case of such a dicotyledon as Casuarina. 



In Fig. 62 are reproduced the essential features of topography of 

 a small branch in this genus. Leaves, as in the case of the conifer- 



