lyS PLANT WORLD. 



are they present in the hypocotyl. They are absent too, 

 from the basal portion of shoots ("retonos") which arise 

 from the root. Another very marked feature is the intrusion 

 of parenchymatous tissue by lateral proliferation into the 

 larger and older canals. In the older stems, the outer cortical 

 canals become stretched to surprising dimensions, and often 

 become wholly or partly filled with tissue. The effect, we 

 may presume, is to close the canals which are about to be cut 

 out by the inner periderm, and so prevent leakage of resin. 



In the hypocotyl the primary canals arise in the 

 endodermis, one opposite and, by virtue of its origin, just 

 outside each of the four primary bundles. Later these be- 

 come secondarily removed to a position further out, this 

 change being accompanied by others involving the establish- 

 ment of a secondary starch-bearing zone of cells. In the 

 root, on the other hand, the primary canals are found to be 

 in two groups of two to three each, each group opposite the 

 primary phloem, and arising, again, in the endodermis. All 

 the secondary canals arise in connection with the secondary 

 bundles, at the outer periphery of the phloem. As Ross 

 observes, there are fewer canals in the root than in the stem, 

 where they arise outside of the endodermis secondarily, which 

 appears not to occur in the root. It may be pointed out here 

 that these relations hold in the main in the mariola {Parthen- 

 iiim inciDiiiui) and in certain herbaceous annual species of the 

 genus. 



The mechanical system, including wood. As pointed 

 out by Dr. Ross, it is very difficult if not impossible to determ- 

 ine the age of a guayule plant by the ring structure of the 

 wood. Annular markings may he detected, but I have not 

 been able to trace them clearly in sections. Ihe suggestion 

 is made, however, that the zone formation in the bast may 

 serve as an index, and this in a rough way appears to be the 

 case. The number of zones is, nevertheless, related rather 

 to the thickness of the stem than to the age, the thickness 

 being related to the number of superimposed branches. 



