Anatomy and Histology. 99 



Stereome. 



The primary stereome arises early in the hypocotyl, as four slender 

 bundles just within the canals, within the outermost part of the primary 

 leptome strands. Occasionally, also, endodermal cells adjacent to the 

 canal may undergo sclerosis, both in the hypocotyl, stem, and leaf (plate 

 25, figs. 5, 6, 11). 



That unmodified pericycle cells lying just within the endodermis 

 and opposite the leptome become sclerosed seems possible (plate 25, fig. 6), 

 but doubtful. I find that the pericycle is quite frequently interrupted 

 (plate 22, fig. 12), in which event the stereome must arise in the primary 

 leptome. Its further development is contributed to chiefly by elongated 

 elements in the leptome, and a few elements are sometimes derived also 

 from the leptome parenchyma. Nearly all the elements (except those of 

 parenchymatous origin) which play this part enlarge greatly (plate 25, 

 fig. 4) and cause marked displacement in the surrounding tissues. Vuille- 

 min (1884a) has described stereome arising, in the Compositae, in the sec- 

 ondary, but not in the primary leptome, in Achillea, Artemisia, Antkemis, 

 and Leontopodium. From my own studies I am forced to the conclusion 

 that this takes place in the primary leptome. 



SECONDARY STRUCTURE. 

 The prophyllonary bundles, above referred to, arise in the intervals 

 between the cotyledonary bundles, before the establishment of inter- 

 fascicular cambium (plate 24, figs. 2 to 5, 12). This, when complete, 

 incloses the cotyledonary hadrome, and there is thus established the basis 

 for the imposition, on the primary stele, of secondary, true stem topog- 

 raphy. It may be pointed out, however, that the cambium does not 

 lay down secondary hadrome in all cases in immediate contact with the 

 primary elements. Thus, in the radii of the median cotyledonary traces 

 (the elements of which do not of course pass beyond the cotyledonary 

 node) secondary hadrome arises which descends from the epicotyl (plate 

 25, figs. 10 to 106). Between these there is frequently a hiatus which 

 delimits them readily to the eye, if the secondary changes have not pro- 

 ceeded too far. Nevertheless, though the morphological separateness of 

 the primary and secondary hadrome -and also leptome is clear, the 

 peculiar topography, the curved outline of the secondary hadrome as 

 seen in transverse section, indicates an as yet entirely unanswered ques- 

 tion as to the immediate cause of this. As it is purposed to compare 

 ecological types, further detail will be considered in the following para- 

 graphs. 



Field Plants. 



The pith in a specimen about 1.8 mm. in diameter displays at an 

 appropriate level two gaps (plate 25, fig. 7), each in the position of a 

 primary medullary ray, containing the primary bundles l constituting the 

 lateral leaf-traces, while its transverse outline still reflects the vascular 

 topography of the primary condition. Surrounding the pith is a closed 

 compact column of hadrome which is broken up radially into broad wedges 



1 These undergo little or no secondary thickening, except in a restricted region 

 below the cotyledonary collar. 



