172 Guayule. 



off a number of branches which distribute themselves in the cortex of the 

 bud. As already said, generally a single branch from the pith-canal oppo- 

 site the bud enters and branches to produce the complement of canals 

 (plate 39, figs, i and 7). 



SECONDARY CANALS IN ROOT, HYPOCOTYL, AND STEM. 



These arise, as described by Ross, from special leptome parenchyma ' 

 derived directly from the cambium, and quite in the same way in all parts 

 of the plant. They are at first flattened radially, opening out later to be- 

 come rounded or even circular in transverse section, and finally becoming 

 again flattened and secondarily distended, in company with the growing 

 (secondary) cortex (plate 22, fig. 13). These canals constitute concentric 

 branching and anastomosing systems, each succeeding zone being a sys- 

 tem separate from all the others. Their appearance in tangential sections 

 (plate 39, fig. 8) recalls the figure published by Tschirsch (1906, p. 11 93) 

 of the canals in wound-tissue in Larix. 



CANALS IN THE PEDUNCLE. 



It has already been pointed out that the inflorescence is terminal; 

 the peduncle is therefore the morphological chief shoot. I have shown 

 that when an axillary bud develops it usually receives one canal from the 

 pith (plate 39, fig. 1). The last bud formed on the chief shoot which ends 

 in a peduncle, however, receives all of the canals from the pith, these being 

 diverted en masse. The peduncle, therefore, contains no medullary canals 

 (plate 39, figs. 2, 3). Primary cortical canals alone occur, there being but 

 very little secondary thickening. 



Exceedingly interesting relations in this regard are displayed by 

 rapidly grown plants (plate 39, figs. 4 to 7). In another chapter two types 

 of guayule have been described, in one of which the sharp delimitation 

 between peduncle and foliage stem is not present. When guayule is irri- 

 gated there frequently results, associated with rapid growth, a tendency 

 of the relatively chief shoot to run out into inflorescence, 2 when otherwise 

 there would be a sharp transition from stem to peduncle, and the upper 

 axillary bud would develop strongly. When the morphological transition 

 is gradual, there is also a correlated anatomical transition, which the long 

 internodes make it possible to analyze. In a specimen examined, as in 

 the normal condition, the peduncle has no pith-canals, but the first inter- 

 node below this has, instead of five, only two, which pass into the upper- 

 most axillary bud. 



The sector of the stem under the peduncle contains much more stere- 

 ome, and the two canals are confined to the sector beneath the axillary 

 bud, while from the basal part of the internode they are absent! Their 

 orientation above is such as to bring them opposite the first and second 

 leaves of the axillary bud; they are, therefore, the canals which give 

 branches to the first two axillary buds of the branch. 



The axillary bud of the second node below the peduncle receives from 

 the stem one canal only of four which are to be found in the internode be- 



1 Secondary leptome-canals have been described in Centrophyilum lanatum 

 (Col, I.e.). 



2 Simulating the normal shoot in P. incanum (mariola). 



