106 Guayule. 



opposite sides of the same section. In the position opposite the first leaf- 

 trace the divisions occur in the second layer; at the other end of the 

 diameter, in the outermost. The periderm figured by Fron and Francois 

 is therefore the leaf absciss layer. Leaf fall in the guayule is consum- 

 mated only slowly, and, as compared with more familiar examples of the 

 temperate regions, is imperfect in its time relations. The layer is not 

 sharply defined, and the disintegration of the tissue is irregular, the result 

 of the uneven and irregular character of the component cells of the 

 absciss layer. 



The epidermis, both of stem and leaves, in the epicotyl is clothed 

 with a single type of trichome found throughout (plate 30, figs. 5 to n). 

 There are two derived kinds, a T-shaped hair predominating, with a few 

 scattered hairs of a type seen in Chrysoma (Lloyd, 1901) and in other 

 Compositae, viz, the whip-hair, but in which, in the guayule, the terminal 

 cell remains undeveloped. The trichome does not, therefore, become 

 flagellate, as in the related species, the mariola (Parthenium incanum] , and 

 in many Compositas (Vesque, 1885). In certain places, as in the axils of 

 the leaves, floral bracts, and corolla, transition forms may be met with, 

 indicating that the two kinds have been derived phylogenetically from a 

 single type. The fact that both are present in different Compositae, but 

 in different ratios, may be used to support the view that the trichome 

 clothing is a character which has been brought about by gradual change 

 and not by the sudden dropping out of one or the other kind. The 

 T-shaped hairs clothe the plant very completely and smoothly, the termi- 

 nal cells all lying very nearly parallel to each other, and to the axis, on 

 the various organs. The density of, the covering varies, however, with the 

 size of the organ, as the individual hairs show no substantial amount of 

 response to varying external conditions. 1 



Before leaving this part of the subject it is necessary to point out 

 that in seedlings in which the stem elongates slowly, as in the field, the 

 primary cortical canals of the hypocotyl behave in a manner which has 

 not been observed in etiolated plants. The two pairs, associated with the 

 median leaf -traces of the two early foliage leaves, instead of passing directly 

 into the petioles, anastomose and then, from the transverse lacuna thus 

 formed, canals pass off to enter the leaves. Other canals have been noted 

 to rise from the lacuna and to pass up into the epicotyl; a reanastomosis 

 within a short distance has been observed (plate 36, figs. 7,8). A section 

 of a field seedling made through the cotyledonary node, or at any level, 

 if the internodes are undeveloped, will almost invariably show widely 

 spreading divarication of one or more of the canals (plate 36, fig. 6). In 

 a word, the canals constitute a branching system, each more or less in 

 communication with the other. 



1 The mechanical conditions in axils of leaves and in the capitula cause super- 

 ficial changes in the shapes of the trichomes. 



