110 Guayule. 



The stereome develops, therefore, within the leptome, 1 and in its defin- 

 itive form a portion of the leptome comes to occupy the volume of the 

 whole. The definitive stereome may be flanked more or less completely 

 by sclerosed leptome parenchyma, and even the adjacent cortical cells, 

 especially in the peduncle, may take on this character. 



With reference to origin, in general terms, I am at variance with 

 Ross, who says on this point: " Durch die Tatigkeit des Kambiums ent- 

 stehen abwechselnde Gruppen von zartwandigen Elementen und von Scle- 

 renchymfasern. In der jiingsten Gruppe der letzteren geht die Verdick- 

 ung der Zellwande erst sehr allmalich vor sich, und in den zartwandigen 

 Schichten zwischen dieser und dem Kambium kommt der Secretkanal 

 zur Ausbildung." I believe that I am not unduly criticizing Ross's state- 

 ment by saying that it is misleading. It would seem more consonant with 

 the facts to say that through the activity of the cambium alternating 

 groups of leptome parenchyma and prosenchyma arise, and that the 

 stereome arises within the latter. The resin-canals arise from two adja- 

 cent tangential layers of the thin-walled parenchyma. 



The change of any particular cells into stereome is not complete 

 before the end of the third season's growth, as nearly as we may judge. 

 This secondary occupation of the leptome by the stereome is particuarly 

 well shown in a preparation made of the cortex of an old stem (plate 32, 

 fig. 2). The sections were treated with xylol so as to extract the rubber, 

 leaving the tissues empty and distinct. The stereome was seen to occupy, 

 with few exceptions, all the space previously occupied by the sieve-tissue. 



ORIGIN OF THE MEDULLARY AND CORTICAL STEREOME. 

 Vuillemin, 1884c, p. 223, has described stereome as arising in the 

 pericycle in the Composite, but he does not show its precise origin nor 

 that of its constituent elements; nor does his description of the leptome 

 (I.e., p. 99) fit the conditions found in Parthenium argentatum. Accord- 

 ing to Vuillemin, the sieve-tubes are of much larger transverse diameter 

 than the companion cells, and this is not true of our plant. There are, 

 however, broad elements with oblique end-walls, 2 intermixed with sieve- 

 tubes and companion-cells to form a melange in which the sieve elements 

 are generally in contact with each other throughout the whole leptome 

 mass, and do not usually form isolated islands, as generally described for 

 the Compositae. These elements have common origin in cambium cells; 

 that is to say, the broad elements and the narrow sieve-tissue elements are 

 of common descent. The broad cells, which later are transformed into 

 stereome, do not, therefore, have a distinct origin. The initial division 

 within the mother-cell may be periclinal or radial, separating a broad ele- 

 ment, destined to become stereomatic, from a similar one, which again 

 divides once or twice, usually twice, to form the sieve-tissue (plate 31, 

 fig. 9). There is but little difference in the transverse diameter of these, 

 the companion-cells being narrowly fusiform and therefore thickest at the 

 middle, while the reverse, of course, is true of the sieve-cells. The broad 

 elements are recognizable both by their size and by their more tenuous 



1 Vuillemin's description, "sur le dos des faisceaux libe'riens," does not apply. 

 2 The " libriform " of Schwendener, 1874. 



