Anatomy and Histology. Ill 



protoplasmic content. When they become stereomatic the first step is the 

 great enlargement of their transverse diameters, their walls being thin 

 except at the angles, which are thickened after the fashion of collenchyma 

 (plate 31, fig. 8). During this phase of change the mutual pressure of the 

 developing stereome and the surrounding cortex results in the radial 

 flattening of the latter, and frequently in a crumpling of the walls in the 

 stereome. The limit of the stereome may readily be seen because of the 

 intercellular spaces between the cortical cells and those of the stereome. 

 Meanwhile the sieve-tubes and companion-cells become displaced and, 

 with sclerosis of the stereome elements, are destroyed, and may only with 

 difficulty be observed at all. Sclerosis of the stereome proceeds radially 

 from without inwardly. The compactness of the stereome, as also its 

 regularity and dimensions, depends upon the previous mode of growth of 

 the leptome as a whole, and is therefore more irregular and of uneven 

 texture, in irrigated plants, or, what amounts to the same thing, in rapidly 

 grown plants. Sclerosis also overtakes some of the adjacent leptome 

 parenchyma and, under certain circumstances, some of the neighboring 

 cortical cells, but is not preceded by their enlargement. 



The stereome in the medulla (plate 3 1 , figs. 6, 7) , which has previously 

 been so referred to for convenience, is, like the above-described leptome- 

 stereome, a constituent of the mestome strand. It arises from elongated 

 elements clustered about the primary hadrome elements, and is the en- 

 doxyle of Briquet (1892), but, in the light of the occurrence of bicollateral 

 bundles in the Chicoriaceae (Vuillemin, 1884a; van Tieghem, 1884), may be 

 susceptible of another interpretation, viz, that it represents the internal 

 leptome in these forms. This explanation is not decreased by the very 

 close analogy between the stereome of the leptome and of the hadrome. 

 In the young condition the tissue which is destined to become stereome is 

 recognizable (plate 31, fig. 6), in transverse section, by the absence of in- 

 tercellular spaces and the somewhat thickened angles, which, during the 

 stretching of the walls previous to sclerosis, become more apparent, as in 

 the case of the leptome-stereome. Interspaces occur in the adjacent pith 

 and in the hadrome parenchym. The tissue, taking the form of an irreg- 

 ular lunate arc in transverse section, is, therefore, while in contact with 

 the hadrome, not to be referred to this without careful consideration. 

 The progress of change into stereome is identical in all respects with the 

 leptome-stereome, and calls for no particular description; this refers also 

 to the mutual displacement of tissues (plate 31, fig. 7). The analogy to 

 the leptome-stereome is strengthened by the circumstance that longitu- 

 dinal divisions may take place in the earliest formed elements, before the 

 final complement of stereome cells is arrived at, though it must be said 

 that these divisions are not of sufficiently frequent occurrence to enable one 

 to see more than a very few at a time. The form of the elements further 

 likens them to the analogous ones in the leptome, being elongated and 

 having slightly inclined end-walls. I am therefore inclined to regard the 

 medullary stereome as a tissue per se with respect to the hadrome, and as 

 having much in common with the stereome of the leptome, so that it 

 would seem to be properly regarded as representing the internal leptome 

 in genera of the Chicoreaceae 



