448 MESSRS. J. PARKIN AND H. H. W. PEARSON ON THE 
amongst the w.ps. Although usually accompanied by a thin 
outer wall, it is not always so—to wit, Oldenlandia Heynei 
(Pl. 11. fig. 4). 
The supposed function of the convexity in concentrating light- 
rays can hardly hold here, for it is difficult to imagine the plants 
of either region requiring more light—rather the reverse. 
(6) Waviness of the Lateral Epidermal Walls. 
Upper surface. 
Out of 42 w.ps., 33 have straight walls, 9 wavy. 
» 40d.ps., 30 % ” 10 ,, 
Lower surface. 
Out of 42 w.ps., 16 have straight walls, 26 wavy. 
» 40dps., 22 ,, 7 18 ,, 
Sinuous lateral walls in the upper epidermis are then about as 
common in one class of plants as in the other, but in the lower 
epidermis they are decidedly commoner amongst the w.ps. 
(7) Stomata. 
w.p-, lower surface, average number per sq. mm. 264. 
d.p., ” ” ” ” ” 305. 
This difference is about the same if both surfaces are taken 
into consideration, notwithstanding that the w.ps. have stomata 
more often on the upper surface than the d.ps. 
w.p., both surfaces together, average number per sq. mm. 309. 
d.p., ” ” ” . - - 356. 
Sixteen w.ps. have stomata fairly numerous on the upper 
surface ; the leaves of all except six are either erect or semi-erect, 
or with movement. 
Eleven d.ps. have them on the upper surface; of which all, 
except two, have erect or semi-erect, or motile leaves. 
Sun-leaves are stated to have more stomata than shade-leaves *. 
On the other hand, Warming ¢ states, quoting authors, that the 
drier the situation the fewer the stomata. The first statement, 
however, refers to the leaves of the same species according as it 
grows in the shade or in the sun, while the second has to do with 
xerophytes collectively compared with other types of plant 
associations. 
* Dufour, Bull. Soc. Bot. France, xxxiii. (1886) pp. 92-95; also Ann. 
Se. Nat., Bot. sér.7, v. (1887) p. 311. 
t Warming, Ockologische Pflanzengeographie, ed. Knoblauch. (1896) p. 193: 
