98 Guayule. 



the paired cotyledonary median bundles becoming somewhat separated. 

 The separation is, I believe, due to the rapid enlargement of the adjacent 

 parenchyma-cells, so that the secondary elements become, in the lower 

 part of the hypocotyl, definitely dissociated, leaving the primary ele- 

 ments occupying the position of the primary hadrome elements of the 

 root. The primary hadrome plate of the root lies, then, in the plane of the 

 cotyledons (Dangeard, I.e., p. 87). In making the approach to the root 

 the leptome masses revolve, two in one direction and two in the other, 

 until they meet, two and two, to form the diametrically opposed leptome 

 masses of the root 1 (plate 24, fig. 2); above, these same leptome masses 

 pass entirely into the cotyledons, with the corresponding hadrome masses, 

 and not into the stem. The continuity of vascular tissues between the 

 stem and root is established secondarily. 



The above account of the structure is incomplete in that the presence 

 of an originally single tracheal vessel, extending from within the cotyle- 

 don downward through the hypocotyl into the root, has not been pointed 

 out. This trachea (trachee primitive of Vuillemin, 1884, p. 183) consti- 

 tutes a center of development, identical in the hypocotyl and tap-root, 

 for the primary hadrome. It is unnecessary to recount the arrangement 

 of hadrome in these organs, but it is pertinent to insist on the initial 

 centripetal formation of new hadrome elements. The dissociation of the 

 hadrome elements in the hypocotyl strictly speaking, only in the upper 

 portion is due, as Vuillemin has stated (1884c, p. 181), to the rapid 

 development of parenchyma, and is analogous to the secondary splitting 

 apart of the wood cylinder in the same organ by the growth of the con- 

 junctiva. In consequence of this interpretation Vuillemin speaks of the 

 paired bundles as "les deux moities du faisceau," which are secondarily 

 separated by "a medullary ray." The peculiar orientation of the paired 

 bundles represented (but frequently not referred to) by many observers 

 (van Tieghem, Gerard, Dangeard, Goldsmith, Ramaley) is thus, properly 

 I believe, explained. 2 



Primary Resin-Canals. 



These arise in the endodermis, as in the root, as a single canal directly 

 opposite each primary leptome strand (plate 25, figs. 1 to 6). The struc- 

 ture of the canal is similar to that in the root, and consists in its definitive 

 form of eight cells in transverse section. The course of development is 

 not so regular as in the primary-root canals, the meatus being ultimately 

 more cylindrical. Not infrequently the earlier divisions do not all take 

 place, so that three instead of four cells line the meatus (plate 25, fig. 2). 

 As these canals pass into the root they pair off, each pair coming to occupy 

 the position already described, namely, opposite each of the two primary 

 leptome bundles. When more than two canals are encountered in this 

 position it is the result of branching. Occasionally both branch either 

 once, or frequently twice, giving rise at length to four or even six canals, 

 though more frequently three or four only occur. 



1 The above account may be applied to Parthenium incanmn, Lactuca, and 

 Helianthus in its main outlines, and is a type, I believe, of wider applicability than 

 usually supposed. 



2 As for the rest of Vuillemin's views, regarding the nature of the hypocotyl 

 and cotyledons, I will say only that they appear to me somewhat strained, and 

 far less in accord with the course of development than those of Dangeard (1889). 



