168 Guayule. 



TOPOGRAPHICAL RELATION OF CORTICAL CANALS. 



The canals of endodermal origin, instead of taking a cortical position, 

 may, in various plants, take a position within the pericycle alternating 

 with the bundles, or opposite the bundles between the leptome and the 

 endodermis. Holm finds such canals in Ambrosia artemisicefolia, though 

 such was supposed to be the case for A. trifida only (Vuillemin, van Tieg- 

 hem). In Eupatorium (Holm, 1908), also, canals occur "outside the lep- 

 tome." The displacement of the canals and accompanying cells of the 

 endodermis to a position nearer the axis appears to have led Vuillemin to 

 draw the conclusion that the endodermis of the stem is superposed on 

 that of the hypocotyl, an inference which, as Dangeard (1889, p. 122) has 

 said, needs confirmation. Vuillemin's figure (I.e., p. 191) is susceptible of 

 a different interpretation. 



In the young epicotyledonary axis in Parthenium incanum, the canals 

 of the cortex are more usually arranged in pairs, flanking the median leaf- 

 traces. This is the permanent arrangement, as, e.g., also in Zinnia (Vuil- 

 lemin) and in Olearia haasii (Col, 1903). It comes about, therefore, that 

 in the definitive stem of this plant the cortical canals are alternate in posi- 

 tion with the bundles. In the guayule they are usually placed on the same 

 radius with the bundles, and stand therefore opposite the leptome. Both 

 of these arrangements occur in various Compositae. 1 



The transition from an alternating position of the canals with respect 

 to the bundles in the epicotyl to the radially opposite position presents 

 an ontogenetic summary of these two conditions characterizing various 

 Compositae in which one or the other arrangement occurs. It may be 

 added, however, that the position opposite the bundle in guayule is not 

 invariable; exceptionally, canals occur opposite medullary rays. This is 

 true of both primary and secondary cortical canals, though Ross states 

 the contrary. 



ANASTOMOSIS. 



Anastomosis and branching frequently occur between the canals of 

 the primary cortical system. The four earliest -formed epicotyledonary 

 canals, which arise in pairs associated with the first and second primordial 

 leaf-traces, are connected, each with the other canal of each pair, by a trans- 

 verse meatus, which lies above the level at which the lateral cotyledonary 

 traces pass out from the axis. This transverse meatus is a prominent fea- 

 ture of the epicotyl, and is frequently the starting-point of several, usually 

 four, canals. Anastomoses in the definitive stem are usually to be found 

 at the nodes, and in stems with very short internodes they are frequently 

 quite numerous. For this reason, in part, the number of primary cortical 

 canals seen in a transverse section varies, as stated by Ross (1908). As 

 the stem thickens (aside from secondary changes) the number of canals 

 increases, so that from 5 to 20, approximately, may be seen (plate 36, fig. 5). 



1 There are very few cortical canals in Parthenium lyratum and in P. hyster- 

 ophorus, and they occur on one or both sides of a bundle, but not opposite to it. 

 Neither do they stand opposite a medullary ray, strictly speaking, though this 

 appears to be the case in P. incanum. In P. arctium Bartlett they are numerous 

 and alternate with the bundles. 



