T. Holm — North American Terrestrial Orchidece. 203 



equipment. The only tissues which exhibit some thickening 

 are the endodermis and the large, central rjarenchyma ; of 

 these the former is, however, only thick-wallecTjust outside the 

 leptome, and only moderately so. The pith is not thickened 

 very much either, but it occupies such a prominent part, that 

 it necessarily contributes a great deal to the toughness of the 

 root. But the other tissues are thin-walled, and the cortex is 

 composed of 15 compact layers, densely filled with starch and 

 some hyphee. The pericambium is continuous and surrounds 

 6 broad rays of hadrome with rather narrow vessels, arranged 

 very regularly in alternation with the large groups of lep- 

 tome, and border on the very prominent, central pith. 



Aplectrum hyemale Nutt. 



The densely hairy epidermis, the hypoderm and cortex are 

 all thin-walled, and the last of these consists of about 9 layers 

 with narrow intercellular spaces ; no starch or hyphse were 

 observed, but bundles of raphides. The endodermis and the 

 continuous pericambium are, also, thin-walled and surround 9 

 broad rays of hadrome, alternating with large, roundish groups 

 of leptome with a central mass of thin-walled pith. 



Liparis lilii folia Rich. 



The very slender roots show a very feeble structure since all 

 the tissues from epidermis to pith are of a very delicate tex- 

 ture. The epidermis bears many long hairs ; the hypoderm is 

 well differentiated from epidermis and cortex by the cells 

 being somewhat stretched radially and almost regularly penta- 

 gonal. The cortex consists of about 8 layers and contains a 

 few hyphge, but no starch ; it seems characteristic of certain 

 species of the genus that some of the cells of the cortex exhibit 

 the same spiral thickening of the wall as is well known from 

 the roots of epiphytic genera, a fact that has already been 

 mentioned by Irmisch.* The endodermis is very thin-walled 

 and shows the spots very plainly ; the pericambium is continuous 

 and surrounds 12 small groups of hadrome, each with a few 

 vessels, and a corresponding number of small groups of leptome, 

 while a large pith occupies the inner portion of the central- 

 cylinder. 



Tipularia discolor Nutt. 



As stated above, the roots of this plant show the remarkable 

 structure of possessing a velamen of 3 to 4 layers inside a thin- 

 walled, very hairy epidermis. However this velamen differs 

 from that of the epiphytic Orchidece by lacking the character- 



* Beiirage zur Biologie und Morphologie der Orchideen, Leipzig, 1853, p. 34. 



