Mac Doug al. — Syrnbiotic Saprophytism. 29 
The endodermis consists of evenly and slightly thickened 
cells, oblong-ovate in cross-section. The fibrovascular bundles 
are twelve to sixteen in number, and are arranged in the 
form of a thin ring. Each bundle shows one to three vessels 
with no centripetal development. The medulla is large, and 
composed of very loosely arranged elements. 
The formation of a third epidermal layer resembling a 
velamen is not easy of interpretation, even in the light of 
Groom's experiments upon the formation of such tissues in 
terrestrial species. The presence of a velamen may be an 
adaptation for aeration, absorption, protection against loss 
of water, or a vestigial character of ancestral species which 
were formerly epiphytic. So far as this last contingency is 
concerned, the entire genus is terrestrial, and the nearest 
epiphytes are so distant that no relationship can be traced. 
An oriental species examined by Groom, Spiranthes australis 
( G . australis ), exhibits similar features as follows (12, p. 205): 
1 Outside is a persistent piliferous layer of one layer of cells, 
the outer walls of which are feebly cuticularized. The walls 
are marked with reticulate cellulose thickenings which run in 
a direction at right angles to the axis of the root. Some 
of the cells grow out into very long slender root-hairs. 
Mycorhizal hyphae penetrate by means of these hairs, and 
by them alone. Within is a typical exodermis with suberized 
walls and passage-cells.’ Groom also found that root-hairs 
were entirely absent from the two-layered epidermal system 
of Lecanorchis malaccensis , and that the outer layer was 
converted into velamen (12, p. 184). 
The same author has pointed out that Grammatophyllum 
speciosum , which grows both as an epiphytic and terrestrial 
plant, develops a velamen more strongly in the latter form. 
It is evident that the only purpose that could be subserved 
by a velamen on terrestrial roots would be that of absorption ; 
an adaptation that would be highly useful to a plant growing 
under conditions of temperature or soil which would destroy 
root-hairs. Furthermore the cells of the velamen would be 
constantly filled with water attracted by capillarity, which 
