THE STEM 193 



place to the herbaceous one under the influence of necessities con- 

 nected with the greater elaborative efficiency of the leaves, so a 

 still further accentuation of foliar efficiency, correlated with a 

 corresponding multiplication of foliar traces, has led to the further 

 modification of the cylinder of the stem, resulting from the necessity 

 of accommodating a very much larger number of foliar traces in the 

 transverse area of the cylinder. This is usually effected by moving 

 the foliar traces from a peripheral position to the medullary region 

 and more rarely by the running of the foliar traces for one or more 

 internodes in the cortex. The multiplication of the conductive 

 strands from the leaves has a further effect in the organization of 

 the bundles which will be discussed in a later paragraph. 



Not only in the arrangement of the fibrovascular bundles do 

 the monocotyledons differ from the dicotyledons. The bundles 

 themselves differ in the mass of monocotyledonous forms from the 

 great majority of the dicotyledonous angiosperms in the fact that 

 cambial activity is absent. Fig. 143 shows the general topography 

 of the bundle in Smilax herbacea, the carrion flower. The fibro- 

 vascular strand is surrounded by a well-marked sclerenchymatous 

 sheath in all probability corresponding to the pericycle in lower 

 forms. It is only rarely that in the group under discussion endo- 

 dermal structures are well developed about the strands in the 

 stem. Generally a continuous external endodermal sheath sur- 

 rounds the bundles in common and no internal structures of this 

 nature can be made out. That the parenchymatous tissue lying 

 between the bundles in the stem belongs to the fundamental system 

 can often clearly be inferred from its histological character where its 

 elements strongly resemble those of the cortex. The internal 

 organization of the bundle is distinguished by the presence of 

 phloem and xylem collaterally disposed in relation to one another. 

 The xylem consists for the most part of vessels and parenchyma, 

 and the vessels are of the porous type. The elements of the wood 

 farthest away from the phloem are typical spiral and ringed proto- 

 xylem. No cambium or zone of growth ordinarily is interposed 

 between the wood and the phloem, and this condition has gained 

 for the fibrovascular strands of the monocotyledons the appellation 

 of closed bundles. It has often been supposed that the closed 



