198 T. Holm — North American Terrestrial Orchidece. 



impossible to offer any satisfactory explanation as to certain 

 facts, for instance, the presence of a velamen in Tipularia 

 discolor, and its absence in Aplectrum, although the nature of 

 the surroundings, the substrate even, is the same in both, and 

 they both are terrestrial. In JBletia verecunda, on the other 

 hand, the development of a similar velamen may be explained 

 as an inherited character since other species of the genus are 

 epiphytic. A similar approach to the epiphytic root-type as 

 demonstrated by certain species of Spiranthes is, also, very 

 puzzling, inasmuch as this genus does not, otherwise, exhibit 

 any analogies in common with epiphytes. 



From these data, it may be readily appreciated that the 

 Orchidece are not to be classified in anything like natural 

 sequence based upon root-structure alone, and we have thought, 

 therefore, that it would be the most convenient to treat the 

 different types of roots by themselves, and regardless of the 

 natural affinities of the genera or species in question. In this 

 way a general idea of the root-structure may be obtained with 

 less difficulty. 



The following types may be distinguished as characteristic 

 of the terrestrial species : 



A. Roots slender with the leptome and hadrome located in 

 one central-cylinder. 



B. Roots tuberous with the leptome and hadrome located in 

 one central-cylinder. 



C. Roots tuberous with several cylinders of leptome and 

 hadrome. 



Of these types the first, A, represents several cases of 

 deviation from the ordinary root-structure by the presence 

 of a velamen for instance (Tipularia, Bletia), by a peculiar 

 striate thickening of cortex (Liparis), and by the- development 

 of a true pith (Calypso, Goody era, Habenaria, etc.). The 

 second type, B, is especially characteristic by reason of its 

 large dimensions, due to numerous layers of cells in cortex and 

 pith, while the third, C, as already indicated, possesses several 

 mestome-cylinders. 



Type A. 



Cypripedium acaule Ait., C. pubescens Willd., 0. guttatum 

 Swtz., G. rnontanum Dough, C. fasciculatum Kellogg, C. 

 Calif ornicum Gray and G. arielinum R. Br. 



In the genus Cypripedium the root-structure comes nearest 

 that of a normal root and may be described as follows : The 

 epidermis is thin-walled in all the species enumerated above, 

 with the exception of C. guttatum, in which the outer cell- 

 wall is slightly thickened ; root-hairs are usually abundant. A 

 thin-walled hypoderm of one layer is, also, noticeable, but is 



