Notes . 
149 
of the velamen, but descend no deeper into the tissue of the root. 
The cells composing the inner layer of the velamen are elongated in 
the direction of the long axis of the root, and possess thin walls which 
show very delicate reticulate thickenings. The inner half of each end 
appears as if filled with a ‘ loosely coherent black-brown substance Y 
which is especially conspicuous outside the passage-cells, because the 
latter are more deeply placed than the other exodermal cells. 
Beneath the velamen lies the suberised epidermoidal layer, or exo- 
dermis, which is made of small passage-cells, rich in protoplasm, and 
of elongated cells with only a thin film of protoplasm in each. Even 
the outer wall of a passage-cell has a thin, external, suberised layer, 
within which is a thick layer of cellulose. The rest of the cortical 
cells, and the vascular cylinder, call for no special description at 
present. The cortical cells are smaller in the ventral half than in 
the dorsal half of the root : so the vascular cylinder comes to lie in an 
excentric position, being much nearer the ventral surface. When 
a root is in close contact with the substratum its dorsi-ventral struc- 
ture is very pronounced : its ventral surface is flattened : it has thin- 
walled, ventral root-hairs : the outer layer of the velamen possesses 
thinner walls than elsewhere : the cortical cells on the same side are 
smaller, and the vascular cylinder is excentric in position. When the 
contact with the support is not so close the flattening of the ventral 
surface does not take place. Finally, when the root is not in contact 
with any support it is radial in structure. Thus the dorsi-ventral 
structure is not induced by the action of light : contact and moisture 
come into play in initiating these structural changes. This last fact 
renders it at any rate possible that the dorsi-ventral structure in many 
orchid-roots is not solely a result of one-sided illumination, as Janc- 
zewski would have us believe. 
It is worthy of note that, in structure and mode of action, this 
two-layered velamen differs as radically from a velamen constituted 
of tracheides, as it does from the ordinary epidermis of a root. 
1 This curious substance found in the innermost layer of the velamen of many 
epiphytic orchids, deserves more minute investigation. Thin sections show that it 
is made up of a mass of fine interlacing filaments forming a sponge-like mass. In 
the young cells these filaments are mainly arranged at right-angles to the inner 
walls of these cells of the velamen and are closely connected therewith. Possibly, 
in nature and mode of origin, this substance is somewhat similar to the intercellular 
rods formed in Marattiaceae. It turns yellow with all iodine-reagents, refuses to 
stain with Hofmann’s blue, and forms in the cell very early. 
