166 Guayule. 



Silybum. Col states that there are six in each group, and it appears from 

 his account that the number of those which pass into the hypocotyl is 

 scarcely reduced. 1 In Parthenium argentatum, however, the number of 

 primary canals is usually not more than four; hence it appears that in the 

 transition zone the number of canals may be doubled. The four pri- 

 mary canals of the hypocotyl pass into the petioles of the cotyledons in 

 pairs, there to end blindly (plate 31, figs. 1 and 2). They do not reach as 

 far as the blade. 



The absence of canals in the blade of the cotyledons is to be noted. 

 According to Vuillemin, the more numerous canals in the hypocotyl of 

 Calendula officinalis pass (in part?) into the cotyledons, on which point 

 Col takes issue. Col's figure of the seedling of this species shows groups 

 of canals opposite four epicotyledonary bundles, and these he identifies 

 with the hypocotyledonary canals, and shows none in association with 

 the paired median-trace bundles of the cotyledons. The position in which 

 Col's drawing shows the canals suggests that they may be the lower ends 

 of the epicotyledonary canals. In many cases, indeed, the true hypoco- 

 tyledonary canals may not follow the primary median bundles even into 

 the petioles on the cotyledons, while in other cases they may. They may, 

 therefore, end blindly in the hypocotyl, by a morphological recedence 

 which Col has cleverly traced for the plant as a whole by his extended 

 comparative study of numerous Compositae. In Parthenium argentatum 

 there are no other canals in the cotyledons (plate 34, figs. 4 to 6). 



PRIMARY CORTICAL CANALS. 



IN SECONDARY ROOTS. 



Primary cortical canals in secondary roots and in those of higher 

 orders arise de novo from the endodermis of the new member. This is 

 brought about by the morphological independence of the endodermis in 

 the roots of different order. Secondary roots are not infrequently triarch 

 (plate 36, fig. 4), and have then three groups of canals, two to four in 

 each group. In roots, either primary or of a higher order, which grow 

 chiefly in length, the canals attain relatively large transverse dimensions, 

 and, with a lacunation of the septa? between them, there arise columns of 

 cells connecting the tangential walls (plate 36, fig. 3). The interpreta- 

 tion has been properly applied by Col (I.e., p. 166) to similar appearances 

 in Solidago. Col's observations do not, however, negative Vuillemin's 

 previous conclusions, "dans les vieux rhizomes d' Arnica montana, etc.," 

 as I point out elsewhere. 



IN THE EPICOTYL AND DEFINITIVE STEM. 



As one ascends the axis the endodermis becomes, as is usually the 

 case, a less definite structure. For this reason it becomes increasingly 

 difficult to determine with precision the exact origin of the primary corti- 



1 Vuillemin (1884a) notes in Silybum a reduction in the number of endodermal 

 root-canals by ending blindly, so that a reduced number pass through the hypo- 

 cotyl into the cotyledons. The question naturally arises whether the reduction 

 in number is not produced by coalescence, as in guayule. 



