BOTANY OF THE CEYLON PATANAS. 441 
all appreciably that of the upper. Gynura Pseudo-china is 
remarkable in having both epidermal layers very deep; in fact, 
according to the table, the lower is slightly the deeper when the 
thickness of the outer wall is excluded. 
(4) Thickness of the Outer Wall of the Lower Epidermis. 
The average is 4°4 p, about 1 » less than that of the upper 
epidermis. When the walls are similar or nearly so in thickness, 
the two surfaces are alike in character, either both glabrous or 
both hairy. Rhodomyrtus tomentosa is an exception—the upper 
surface is smooth, while the lower is covered with fine tomentum, 
yet the outer walls of the two epidermal layers do not differ 
much in thickness; this, however, is an erect leaf. In a few 
instances the lower wall may actually be thicker than the upper. 
This is evident in Bupleurum mucronatum (Pl. 11. fig. 1) and 
Heptapleurum stellatum. 
The relation between the hairiness and the thickness of the 
wall is shown rather more strikingly than in the case of the 
upper epidermis. Thirty-six of these plants have a hairy lower 
surface with an average thickness of wall of 2°6 »; and forty-six 
a glabrous surface with thickness of 5°8 p. A correspondence 
between the thickness of the cuticle and the presence or absence 
of hairs has been observed by Kearney * in the plants of the 
“ Dismal Swamp ” region of Virginia. 
Two features of the epidermal cells not included in the table have 
been observed in detail, viz. :—the convexity of the outer wall, 
and the waviness of the lateral walls as seen in the surface view. 
(5) Convewity of the Outer Wall of the Upper Epidermal Cells. 
The outer walls are frequently convex (Pl. 11. figs. 2 & 4 and 
Pl. 12. fig. 4) ; the surface in consequence is not level, but 
consists of a series of slight bulges, each corresponding to a 
single epidermal cell. Forty-five of these plants have more or 
less these arched walls, and the remainder, thirty-seven, have 
straight or nearly straight ones. This feature has been compared 
with the thickness of the outer wall on the one hand, and with 
the depth of the epidermal cell on the other. 
Leaves with With arched 
straight walls. walls, 
Average thickness of outer wall .. 6°8 4°6 p 
Average depth of cell..........-. 30 32 
* Kearney, Contr. from the U.S. Nat. Herb. v, no. 6 (1901), p. 389. 
