210 T. Holm— North American Terrestrial Orehidew. 



est part of the stolon (by 7) contains the bud, and shows, 

 besides, the thirteen small mestome-cylinders, already observed 

 in the section 5, arranged in two arches parallel with the ven- 

 tral face of the stolon, and with the points of the arches meet- 

 ing near the central-cylinder of the bud-axis. The large 

 mestome-cylinder is still visible at the line 8 and a little below, 

 but disappears at 9 ; from here the small cylinders have 

 increased to seventeen, most of which are arranged near the 

 periphery with a few scattered nearer the center. These peri- 

 pheral mestome-cylinders are quite small and show the same 

 structure as described above ; the interior are somewhat larger, 

 but show, nevertheless, an identical structure. 



If we now dissect the part of the rhizome located by the 

 line 11, we perceive the structure that has been described so 

 often as characteristic of the " tubers " of Ophrydew : a large 

 number (23 in this case) of mestome-cylinders, each with a 

 special endodermis and pericambium, and arranged, but not 

 very regularly, in two zones ; the distinction between the two 

 epidermal layers (the dorsal and the ventral) has vanished, and 

 the brownish, conical and tuberous body is now covered all 

 around by papillae and very short root-hairs. 



It would thus appear as if at least the upper portion of the 

 tuberous body of the rhizome of Platanthera dilatata and 

 the other North American Ophrydew, which we have examined, 

 is composed of elements pertaining to a stolon, a bud and 

 some roots, instead of being simply a root, a concrescence of 

 several roots or finally a swollen bud-axis. But in offering our 

 support to this explanation, pronounced so many years ago by 

 the French botanist, we are well aware of the difficulty which 

 confronts any investigator who deals with organs that remain 

 in their primordial stage and which are not known to occur 

 otherwise, as the supposed secondary roots of Ophrydew. 



If we had only succeeded in detecting some distinctions in 

 the cortical parenchyma of the stolon, when the supposed roots 

 make their first appearance, so as to be enabled to discriminate 

 between the cortex of the stolon and that of the roots, then 

 there would have been more substantial proof in explaining 

 this organ as a concrescence of roots and stem. But the only 

 distinction which we have noticed depends upon the variation 

 expressed in the epidermal structure, the constant dorsiven- 

 trality of the stolon from the first appearance of the secondary 

 roots, and the structure of the small mestome-cylinders being 

 identical with more slender roots of the same species. Of 

 course the word "concrescence" is somewhat misapplied in 

 this particular instance, since these secondary roots have never 

 been observed to be free, not even at the youngest stage of the 

 stolon or of the bud. But otherwise our definition of the 



