348 MR. M. C. POTTER ON THE 
SARCOCEPHALUS. 
One of the chief characteristics of the Order Rubiaceæ is the 
presence of stipules, and hence in this Order one naturally looks 
for some examples of their use as protective organs. 
The example we have chosen for description is Sarcocephalus 
esculentus, Afz. In this genus, and in many genera of the 
Rubiaceæ, at each node two leaves opposite to each other are 
borne, and also two stipules alternating with them, and these 
stipules successively cover over the growing-point (Pl. XLVII. 
fig. 6,A). This is better seen when one stipule has been removed 
(fig. 6, B) and the growing-point deprived of half its protective 
covering. Each pair of leaves is in this manner covered up 
until they are sufficiently developed to be allowed to expand. The 
stipules are deciduous, and fall off when their protective function 
is accomplished. 
2. Protection by the Position assumed by Young Leaves. 
One of the external harmful conditions against which young 
leaves have to be guarded is, as we have already seen, the heat 
of the sun. Now the amount of heat received by a given surface 
from the sun varies directly as the sine of the angle which the 
incident heat-rays make with the given surface; so when a leaf 
is placed horizontally and the sun is vertically over it, the leaf 
receives the maximum amount of heat (because sin 90^—1); and 
in the Tropies one notices a great many leaves which are burnt 
or scorehed by being placed in a horizontal position. But, on 
the other hand, when a leaf is placed vertically and the sun is 
directly over it, the leaf receives the minimum amount of heat 
(because sin 0°=0). Hence we find that many leaves, when they 
are young and still growing, are placed in a vertical position. 
The arrangement above explained to guard a leaf from excessive 
heat can be well studied in many of the Palms; and from among 
this Order we will select the Cocoa-nut (Cocos nucifera) for 
illustration. à 
A fully developed Cocoa-nut leaf is pinnate, with each pinna 
V-shaped, the apex of the V pointing upwards; but when quite 
young each pinna is folded, so that its halves lie flat upon each 
other, and all the pinne are closely pressed together upon each 
side of the rhachis, and the whole projects vertically upwards like 
a pole from the apex of the tree. In this condition the leaf 
