112 Guayule. 



Precisely these relations occur, to all appearances, in certain of the 

 Boraginaceae, e.g., Symphytum tuberosum, Nonnea alba, Omphalodia lini- 

 folia, etc. (Jodin, 1902). Concerning the leptome, Jodin says, after speak- 

 ing of the disappearance by crushing of the sieve and companion elements 

 ("les primaires tubes cribles"): 



En mfime temps que s'accroissent les elements libe'riens primaires, on peut 

 assister, dans certains genres a un ^paississement notable de leurs parois * * * * 

 Dans d'autres cas, cet dpaississement est tres faible ou meme n'a pas lieu * * * 

 (I.e., p. 308.) 



But no such thickening takes place in the secondary leptome. Appar- 

 ently the thickening of which Jodin speaks goes no further. He does not 

 trace the precise origin of the cells with thickened walls. 



As to the medullary stereome, he says little, but his figures show very 

 clearly the earlier, prestereomatic condition which I have shown in my 

 own figure (plate 31, fig. 6). To quote again: 



Nous aurons peu de choses a dire de la moelle; nous avons eu occasion de 

 parler, a propos des faisceaux du bois, de la zone peYimdullaire, et des rayons 

 medullaires. La region medullaire proprement dite se distingue par la taille de 

 ses cellules qui sont arrondies en coupe transversale, et qui laissent entre elles de 

 nombreux meats triangulaires. (/. c, p. 322.) 



This author, it is seen, points out the same distinctions between the 

 perimedullary zone and pith which I have already made. From this com- 

 parison between the guayule and the borages it seems clear that we are 

 dealing with the same behavior, with the very interesting distinction that 

 in the guayule the histological differentiation of the fibers proceeds to 

 completion, while in the plants studied by Jodin they are arrested in their 

 course of development. This appears to be connected with the herba- 

 ceous character of the stems in the Boraginaceae. 



In this connection, Schwendener's observations on certain Composi- 

 tae are of particular interest: 



Im Phloem der grosseren Aster und Solidago Formen, * * * kommen inner- 

 halb der starken primaren Bastbiindel kleine secundare Gruppen mechanischer 

 Zellen zur Entwickelung, welche zum Theil mit den kurzesten Libriformzellen, 

 die uberhaupt vorkommen, ubereinstimmen, und jedenfalls durchgehend vom 

 typischen Bast verschieden sind. Die Lange diesen Zellen variert zwischen 150- 

 300 Mik.; die Kurzesten erreichen oft nur bis 80 Mik. Dazu kommt, dass die 

 nebeneinander liegenden schiefen Querwande ahnliche Zick-zacklinien bilden, wie 

 sie sonst nur in kurzzelligen Libriform vorzukommen pflegen. Bei Aster bilden sie 

 im Querschnitt netzformige anastomosirende Bilden, zwischen denen ein parenchy- 

 matisches Cambiform stellenweise mit deutlichen Siebrohren, eingebettet liegt. 1 



This distinction made by Schwendener between the sclerosed element 

 of the "phloem" and typical bast applies throughout to Parthenium ar- 

 gentatum. This plant, however, differs in the distribution of the sclerosed 

 elements, forming as they do dense masses occupying the space previously 

 occupied by the whole of the leptome and its associated libriform. 



Schwendener, however, appears to assume the independent origin 

 of the libriform cells in the leptome, and it is on this point that I advance 

 the view that they have a common origin with the sieve and companion 



1 Schwendener, 1874, p. 152. I have not had access to this paper. 



