140 Curtis . — The Anatomy of the Six Epiphytic 
The leaf has a central and two smaller lateral depressions. Near the 
edge the lower surface slopes gradually towards the upper. No isolated 
strands of sclerenchyma are present. The rigidity of the leaf is due to the 
presence of strikingly large vascular sheaths whose outline in transverse 
section resembles roughly that of the figure 8. The largest strand lies 
in the centre of each leaf-half. The fibres are small and have many stria- 
tions except where phloem and xylem meet. Surrounding each sheath is 
a single ling of small hexagonal thin-walled cells containing numerous 
chlorophyll granules. Between the vascular strands the cells of the assimila- 
tory tissue are much elongated in transverse section, while above and below 
this region they are more nearly oval. Numerous intercellular spaces are 
present in the lower half of the leaf. Club-shaped cells, attached to small 
basal cells, occur sparingly in the upper epidermis and infrequently in the 
lower. The epidermal cells adjoining the basal cell are smaller than usual, 
and their walls are distinguished by the presence of large elongated pits. 
The inner and outer walls of the two epidermes undergo pectic mucilaginiza- 
tion. The cells of the upper are unusually large. Minute round pits stud 
the walls, especially those separating two epidermal elements. Above 
the midrib the cells, which are elongated in the direction perpendicular 
to the leaf surface, resemble the epidermis in the mucilaginous nature of 
their walls, in their vacuolated protoplasm, and in the size and shape 
of their pits. 
Bulbophyllum pygmaeum, Lindl. Gen. et Sp. Orch. 58. 
Minute, forming densely matted carpets on the trunks of trees or 
on rocks. Pseudo-bulbs § in. in diameter, globose, glabrous. Leaves 
solitary on the pseudo-bulbs, springing from a minute circular sheath J-J in. 
long, linear-oblong, obtuse, very thick and coriaceous, grooved down the 
middle and minutely echinulate above, longitudinally nerved beneath. 
Peduncles solitary from the base of the pseudo-bulbs, -|-| in. long, 1 -flowered. 
Flowers very minute, whitish. 
The root is very small (PL X, Fig. 57). The velamen consists of 
a single row of regular, comparatively large cells lacking thickening strands. 
The inner tangential walls are thick and cutinized ; the radial are of cellulose 
and diminish in thickness towards the surface of the root, where the outer 
tangential walls are very thin. The thickened exodermal cells have cutinized 
striated walls and the passage cells are often of equal size. The cells of the 
cortex have no spiral thickening strands. They are four to six deep, are 
small at the inner and outer limits, but in the centre are large and oval with 
distinct intercellular spaces. The longitudinal walls of the row adjoining the 
endodermis are reticulately pitted. Raphides are present in certain cells of 
the second layer from the exodermis. Chlorophyll occurs throughout the 
cortex, but is more abundant in the small cells adjoining the exodermal 
