THE HERBACEOUS DICOTYLEDONS 



393 



level to that in Fig. 2700. It is thus evident that, if the woody 

 cylinder is thin enough, the leaf trace will correspond to it in 

 thickness, and that as a result the subtending or confronting 

 parenchyma, so prominent a feature of the topography of the leaf 

 trace in the cylinder of Leea, Casuarina, the oak, and other woody 

 stems with compound rays, is plainly absent. The comparison 

 of very slender and thicker annual stems of the vine with those 



a b 



FIG. 270. a, transverse section of foliar ray in Vitis in the region of the node 

 b, transverse section of foliar ray below the node. 



of the shrubby Leea makes it clear that an extreme thinning of 

 the axis brings with it a condition of organization in which the 

 storage parenchyma no longer surrounds the foliar trace, but 

 merely flanks it on either hand. In climbing or herbaceous axes, 

 in which this slenderness has become a fixed feature, some com- 

 pensation for the loss of subtending or confronting storage tissues 

 is provided by the great lengthening of the flanking rays, which 

 often extend, as was pointed out by Strasburger over a quarter 

 of a century ago, through one or more internodes. It will be 

 obvious from the facts introduced in the present paragraph that 



