.4 natomy and Histology. 1 1 3 



cells. After arriving at this conclusion, I found that Servettaz (1909, p. 

 232) had already done so with respect to certain of the Eleagnaceae. The 

 close resemblance in the behavior of the medullary mass of stereome to 

 that of the leptome forces criticism of this view to the effect that the 

 analogy which I have drawn is based on the origin of the stereome in 

 the hadrome and leptome from libriform of an identical mode of origin 

 on either side of the cambium. This view, while admittedly possible, does 

 not agree with my observations, and it is hoped that further research will 

 bring evidence to light which will show which view is correct. 



The secondary resin-canals, when fully formed, are composed of an 

 endothelium backed usually by one row of leptome parenchyma (plate 29, 

 figs. 1,2). In transverse outline, after full development, they are rounded, 

 but gradually become compressed radially as they pass outward toward 

 the bark. The youngest ones measure upward of 0.2 mm. in tangential 

 diameter, and grow in size till, at the outer part of the living cortex, they 

 mav measure, in a cortex 5 mm. thick, over a millimeter tangentially, 

 and with a width a third of this. The secreting cells undergo more or less 

 periclinal divisions (with reference to the axis of the canal), producing 

 sometimes two to three layers of cells of endothelial origin. The resin- 

 canals at length frequently become partly or completely closed by an 

 ingrowth of tissue (Lloyd, 19086) of the same character as the cortex and 

 forming an interesting analogy to tracheal plugs (tyloses). These I call 

 pseudotyloses (plate 32) . The cells of the pseudotyloses at length become 

 filled with rubber and continue in a living condition somewhat longer 

 than the surrounding cortical tissue, retaining their normal appearance 

 when the cortical cells toward the outside of the stem have passed over 

 into suber. These parenchymatous plugs are not confined to the very old 

 tissues, but may be found also in young stems and roots, 1 though less 

 frequently. Occasionally the medullary canals, in old plants at any rate, 

 become partially plugged in the same manner (plate 3 2 , fig. 3) . In addition 

 to these outgrowths, resembling roughly a bunch of grapes, one frequently 

 finds trichome-like structures, sometimes projecting from the walls and 

 also from the plug-tissue (plate 32, figs. 1,6). Somewhat similar appear- 

 ances have been observed by Col, and to this I shall call attention again. 

 In this connection, however, I feel inclined not to agree with this author 

 in his criticism of Vuillemin, who recorded observing structures which he 

 called " poils glanduleux " in the canals in old rhizomes of Arnica montana 

 (Col, 1903, p. 166). I suspect that these "poils glanduleux" are the same 

 structures as those which I have called pseudotyloses. 



The pith undergoes a considerable amount of secondary enlarge- 

 ment, so that in a stem 2.5 cm. in diameter, in which it may still be found 

 in a living condition, its diameter is between 3 and 4 mm. and is irregular 

 in outline. The medullary stereome does not receive any secondary accre- 

 tion, but the growth of the inner part of the parenchyma rays concomitant 

 with that of the pith, between the edges of the xylem wedges and the 

 flanking stereome, results in the periclinal separation of these. Sometimes 

 one may find that cells near the periphery of the pith have undergone a 



1 I have observed them in the primary canals (plate 32, fig. 7). 



