TRANSACTIONS OF SECTION D. 1085. 
varying in length in the different species, Above this, receding from the growing 
apex the epidermis is of a white colour. 
A horizontal section shows the aeriai root to consist of three concentric rings 
of tissue. 
The outer or velamen consists of a varying number of layers of cells. In 
Oncidium about ten, in Aerides two or three. These cells are polygonal, closely. 
packed together, contain no chlorophyll, man y are filled with air. These give the 
velamen its white colour and render it hygroscopic. Through these layers of the- 
velamen many spiral vessels are found. 
The middle ring is bounded by two layers of compact cells, the outer endo-- 
derm and the inner endoderm. The outer endoderm consists of a row of thick- 
walled compact cells, In this row at intervals are thin-walled cells leading from: 
the velamen to the central ring. The cells between the outer and the inner 
endoderm are large with chlorophyll granules, and contain the usual cell contents, 
The number of layers of these cells varies. I have found most in Aerides, fewest 
in Dendrobium. The central ring very clearly marked off from the middle ring is 
composed of a dense mass of fibro-vascular tissue. Next the inner endoderm the 
xylem and the phloem portions alternate. The thick-walled cells of the inner 
endoderm lie opposite the phloem portion, and the thin-walled cells opposite the 
xylem portion. These gradually merge into the large cells of the medulla, They 
do not play any important part in the investigation. 
In which of these rings are the mycelia found? In all the species I have 
examined they occur invariably in the middle ring. Only in the cells with chloro- 
phyll have I found mycelia; where the mycelia are most abundant the protoplasm 
of the cells is least. These cells are comparatively empty. Of the various rows of 
cells in the middle ring or zone the central contain most mycelia. I have never 
seen a single thread attempt to pass by means of the thin-walled cells of the 
inner endoderm to the central zone. Nor have I ever seen these threads in the act 
of penetrating the thin-walled cells of the outer endoderm. In some species of 
Cattleya, e.g. Cattleya Laurenceiana, where the development of mycelia was very 
strong—the central zone being nearly filled by them—I have never found them 
penetrating these thin-walled stoma-like cells. 
The mycelia are never found in the green terminal growing apex of the root,. 
but are first found just where the velamen with its hygroscopic cells begins. I have 
traced them from this point all through the aerial root as far as the pseudobulb. 
This they haye never entered in any plant I have examined, nor have I found any 
trace of mycelia in any other part of the plant—flower, gynandrium, or leaf, 
gain, in cells which have little or no protoplasm, the mycelia are strongest 
and most abundant, while in those cells with most protoplasm, which often gathers. 
in lumps, the mycelia are thinner and less vigorous, 
Constitution of Mycelia. 
In some cases the hyphe, for such I consider that part of the fungus which 
branches and anastomoses, are found towards the posterior portion of the root, 
while the mycelium portion with few branching threads is found nearer the 
anterior portion of the root. In Cattleya trienne the branches are large, thick, 
and with node-like swellings like the buckle-shaped swellings on the hyphe of 
Basidiomycetes. Sections of the root containing these swellings were sown in a 
suitable medium. From these swellings came only their mycelium-like threads. 
In no instance have I been so fortunate as to find any fructification, either in the 
root or arising from sections sown in a suitable nourishing medium. From the 
varied appearance of the threads, however, it may be concluded that the fungi 
belong to different families, 
Relation of the Mycelia to Cell-wall and Cell-contents. 
The appearance of the cells into which the mycelia have penetrated is not 
essentially changed by the presence of the fungus, “Near the growing apex of the 
root where the mycelia are first found the cells remain unaltered, nucleus and 
