170 Guayule. 



from the regular occurrence of live canals, the primary number in the 

 growing stem apex. In seedlings with short internodes the canals appear, 

 of course, nearer the hypocotyl. In a field seedling 3 cm. tall, with two 

 dozen or more nodes, I found one canal at 5 mm. above the hypocotyl. 

 The next section cut had one. Similarly in an irrigated seedling with 

 short internodes. 



The absence of pith-canals in the epicotyl suggests a primitive al- 

 liance with those tubuliflorous forms in which canals are entirely absent 

 from the pith. 



IN THE DEFINITIVE STEM. 



At the growing apex within 0.5 mm. one finds a strictly primary ar- 

 rangement of these canals. There are five, one opposite each orthosti- 

 chy. 1 In a slowly growing stem, however, in which the nodes are crowded 

 upon each other, through frequent branching and anastomosis, the num- 

 ber seen will vary usually between three and six. The union and separa- 

 tion of the canals is associated with the formation of large lacunae giving 

 off large passages of irregular shape, but on the whole running longitudi- 

 nally. In a single section, therefore, one may count as many as a dozen 

 canals, and nearby as few as three or four. In rapidly growing shoots the 

 anastomoses and branches are not so apparent, though they occur here 

 also. From the canal nearest to it each bud receives normally a single 

 branch, which, itself branching after entering the bud, increases till the 

 complement is reached. Pith-canals do not enter the leaf. 



TOPOGRAPHIC RELATIONS OF MEDULLARY CANALS. 



Although the primary number of pith-canals is more or less masked 

 by branching and anastomosis, as already mentioned, a study of the on- 

 togeny of the stem can not fail to show that five is the primary number 

 (plate 38, fig. i),and further, that they arise in the same order as the leaves 

 and, therefore, buds. These relations are seen best in growing tips of stems 

 of not too slow growth, or in seedlings, just above the levels at which the 

 pith-canals first come in. It is also evident from the positions taken by 

 the solitary canals which appear in the epicotyl before the full complement 

 is established. 



The very frequent anastomosis and divarication, coupled with the 

 transverse expansion of the canals, give rise to a great many columnar, 

 trichome-like structures, already alluded to. They lie approximately 

 in radial planes, and can be explained only as imperforate longitudinal 

 septae (plate 32, figs. 1,6). 



In older stems the breaking down of the pith results in the opening of 

 the resin-canals, except when plugged by pseudotyloses. There results a 

 downward filtration of resin which finds its way into the central zones of 

 the old wood. This often becomes richly impregnated with resin, though 

 primarily it contains none at all. In this way the resin-content of old 

 wood, shown by chemical methods, is to be accounted for (Lloyd, 1909). 



1 In the pith of Cynara cardnncula (Col, 1903) 5 to 10 canals occur; in 

 Parthenium hysteropliorus I count about 20; in P. lyratum about 12. 



