196 



THE ANATOMY OF WOODY PLANTS 



tufted leaves, the amphivasal condition becomes continuous. 

 Fibrovascular strands of this type are, in fact, a common feature 

 of organization of the subterranean regions of many monocoty- 

 ledons, even when they are absent in the stem. This situation is 

 well exemplified by the orchids and Iridaceae. In Gramineae and 

 Cyperaceae, on the other hand, amphivasal strands are present in 

 the somewhat remote nodes of the annual flowering stems, but 



disappear in the internodes, 

 and a similar situation is pres- 

 ent in the perennial subterra- 

 nean stem in case the nodes 

 are not crowded. In the latter 

 condition the amphivasal state 

 becomes continuous. In the 

 true palms (Principes) and in 

 many Scitamineae the amphi- 

 vasal bundle seems to have 

 entirely disappeared, although 

 this is a matter for further 

 investigation. There seems to 

 be little doubt that the forma- 

 tion of amphivasal bundles in the region of the nodes is an 

 original feature of organization of the monocotyledons, and that 

 it persists very strongly in the perennial subterranean stem or 

 rootstock, but may entirely disappear in the annual stem. The 

 amphivasal condition of the bundles seems to be definitely cor- 

 related with the numerous leaf traces passing into the stem at the 

 node in the monocotyledonous angiosperms. This hypothesis of 

 its origin is supported by the fact that in herbaceous dicotyledons 

 with numerous leaf traces the occurrence of amphivasal bundles 

 at the node can frequently be observed. The Umbelliferae may be 

 cited as an excellent example of this condition. 



The consideration of the stem has now been completed, and 

 it will be advantageous in conclusion to indicate the main tendencies 

 manifested in its course of evolution from lower to higher forms. 

 It has been noted that the first important phase in the evolution 

 is connected with the primary structures of the fibrovascular region. 



FIG. 145. Amphivasal concentric 

 bundle of rootstock of Smilax. 



