T. Holm — North American Terrestrial Orchidece. 207 



however, depends merely upon the number and relative size of 

 the mestome-cylinders, there being 4, 2 large and 2 small, in 

 P. orbiculata, but 8, and all very small, in the other. Besides 

 this variation as to size, their arrangement is, also, quite dis- 

 tinct, since they are located in one ring in P. orbiculata, but 

 in two in the other species. Otherwise the structure is identi- 

 cal ; the epidermis, hypoderm, cortex, endodermis and pericam- 

 bium are all thin-walled, and deposits of starch besides raphides 

 were observed in the cortex of P. orhiculata, hyphse, on the 

 other hand, in P. Hookerii. Moreover there are noticed 15 

 short rays of hadrome in the large cylinders of P. orhiculata, 

 but only 5 in those of P. Hookerii. A central pith was 

 observed in each of these mestome-cylinders and of both 

 species. 



The more slender roots show the same structure as the 

 tuberous, described above, but they contain a correspondingly 

 small number of mestome-cylinders, viz : 3 to 4 in P. Hookerii, 

 and only 2 in P. orbiculata. 



In the remaining species of Platanthera, which we have 

 examined : P. dilatata (Gray), hyperborea (Lindl.), ciliaris 

 (R. Br.), psychodes (Gray), cristata (R. Br.) and tridentata 

 (Hook.), the roots show an almost identical structure, since the 

 principal difference observable mainly consists in their relative 

 size, their length, thickness and corresponding number of 

 mestome-cylinders, characters of no great importance when we 

 bear in mind the fact, that the tuberous development of such 

 roots is extremely variable and often depending upon certain 

 conditions of the substrate or upon the individual strength of 

 the specimen. 



In beginning with the tuberous roots, the epidermis is quite 

 hairy in some species, but merely papillose in others, for 

 instance P. ciliaris and P. psychodes ; this covering with 

 hairs is especially well marked in specimens from Sphagnurn- 

 bogs. A hypoderm of a single layer is generally present, but 

 seems to lack P. dilatata. The cortex is always thin-walled 

 and contains starch, but the number of layers is very variable 

 even in specimens of the same species ; hyphse were observed 

 in all the species except P. ciliaris. 



The innermost portion of the root is occupied by a large 

 parenchymatic tissue, which, also, contains starch and which is 

 hardly to be distinguished from the cortex ; sphserocrystals 

 were observed in great abundance in P. ciliaris and cristata. 

 The mestome-cylinders occur, sometimes, in several more or 

 less concentric rings, but are mostly somewhat irregularly 

 scattered, especially when their number is very large, as in the 

 thickest roots of P. ciliaris. Their number and relative 

 development is variable, but they contain usually from 1 to 5 



