206 T. Holm — North American Terrestrial Orchidece. 



nate with a corresponding number of leptomatic groups, while 

 a broad pith occupies the inner portion of the cylinder. 



The same structure is to be observed in the slender roots of 

 the same rhizome, with the only exception that these contain 

 only one, central mestome-cylinder, the elements of which 

 correspond well with those of the tuberous root, there being 

 about 15 short, hadromatic rays and small groups of leptome 

 surrounding a large, central pith. 



Platanthera. 

 In North America the genus is exceedingly well represented, 

 and occurs with several very distinct types, distinct not only 

 in respect to their flowers, but also in regard to their vegetative 

 organs. The slender, creeping rhizome of P. rotundifolia 

 Lindl. is provided with several slender roots, the structure of 

 which is so near that of P. obtusata Lindl., that they may be 

 treated together. But in all the other species of the genus 

 examined, the roots, especially the one beneath the hibernating 

 bud, are more or less tuberous, and exhibit a structure that is 

 nearly identical with that of the other secondary, but more 

 slender, roots of the same rhizome. 



P. rotundifolia Lindl. and P. obtusata Lindl. 

 Characteristic of the roots of these species is the sparingly 

 hairy epidermis and the lack of any well defined hypoderm. 

 The cortex is thin-walled in both, quite compact in P. obtusata, 

 but rather open and not so broad in the other. Large deposits 

 of starch besides hyphse were noticed in P. obtusata, but only 

 hyphse in P. rotundifolia. Two mestome-cylinders of equal 

 diameter are imbedded in the cortex near the center of the 

 root in P. obtusata, while there are two large and one much 

 smaller in the other. These mestome-cylinders are, thus, 

 separated from each other by some strata of parenchyma, 

 which may be properly defined as pertaining to the cortex, 

 with which it agrees in regard to structure. Each of these 

 mestome-cylinders has a thin-walled endodermis and pericam- 

 bium, the latter being continuous in P. obtusata. The rays 

 of hadrome (3 in P. obtusata, 1 to 5 in P. rotundifolia) are 

 very short and consist of but a few vessels, which, together 

 with small groups of leptome, border inward on a thin-walled 

 pith, which is very prominent in P. rotundifolia, but rather 

 inconspicuous in the other species. 



P. orbiculata (Torr.) and P. Hookerii (Torr.). 

 Habitually these species are yery much alike and very dis- 

 tinct from the other North American species of the genus ; 

 their root-structure is somewhat different. This difference, 



