412 THE ANATOMY OF WOODY PLANTS 



In the stem, however, particularly in the subterranean axis, the 

 collateral type gives place to a concentric condition in which 

 phloem is completely surrounded by xylem. This modification 

 is known as amphivasal, to distinguish it from the amphicribral 

 concentric strands of the Filicales and certain lower gymnosperms. 

 The concentric strands of the monocotyledons present themselves 

 in a very interesting fashion in the grasses and sedges. Here, in 

 the reproductive axes, amphivasal bundles are very numerous 

 in the nodal regions, where the entering of many foliar traces 

 produces a marked degree of crowding and disturbance. The 

 amphivasal bundle, in fact, seems to have originated as a con- 

 sequence of the multiplication of foliar traces in the nodal 

 regions of monocotyledonous stems. This hypothesis of the ori- 

 gin of the amphivasal strand is justified by a consideration of 

 parallel conditions exemplified in the organization of the axis 

 in certain dicotyledons. In many instances where the foliar 

 traces are numerous in the nodal region of herbaceous dicotyledons, 

 these become amphivasal in their structure. This organization, 

 for example, is frequently seen in the Araliaceae and Umbelliferae. 

 In the annual stems of the mass of monocotyledons, whether 

 leafy or scapose, amphivasal bundles are ordinarily absent. When 

 the anatomical structure of the perennial subterranean axis which 

 is often found in the monocotyledons is examined, it very generally 

 presents amphivasal bundles in great abundance and not by any 

 means confined to the nodal regions, even when as less rarely 

 happens the nodes are not closely approximated. The common 

 occurrence of amphivasal strands in the rhizomes of monocoty- 

 ledonous stems is no doubt primarily related to the crowded and 

 tufted character of the leaves which results in the strong approxi- 

 mation of the nodes with the consequent multiplication of amphi- 

 vasal strands. 



It is highly probable that the glumaceous representatives of 

 the monocotyledons represent somewhat primitive conditions in 

 the stock, for here both reproductive and anatomical data seem 

 to harmonize in indicating a low systematic position for both 

 grasses and sedges. It is, moreover, probable that the Juncaceae. 

 which in anatomical organization agree very closely with the 



