THE LEAF. 
65 
“positive heliotropism.” Those parts of plants that 
turn away from the light, as the aerial roots of the ivy, 
are said to possess “ negative heliotropism.” 
(b) FUNCTIONS OF THE LEAF. 
The principal functions of the leaf are C0 2 assimila- 
tion, transpiration and respiration. C0 2 assimilation is 
carried on, as we have already seen, by the chloraplas- 
tids, oxygen being formed as a by-product ; transpiration 
is the giving off of water (through water-pores), or 
watery vapor (through the stomata), which has been 
absorbed by the root hairs and transported through the 
tissues of the root, stem and leaf ; the process of breath- 
ing, or respiration, consists in the taking in of oxygen 
and giving off of carbon dioxide. These several func- 
tions are, however, not confined to the leaf alone, but 
are carried on by all the green portions of the plant. 
(c) VENATION OF LEAVES. 
A more or less close examination of the leaf shows 
it to possess a number of distinct markings, which are 
called veins or nerves. The vein traversing the center 
of the leaf from base to apex is usually more promi- 
nent than the others, and is called the midrib; if we 
compare the venation of the leaf of lily of the valley, 
or Veratrum viride, with that of the oak or pansy, it is 
observed that in the former the veins all run parallel 
to the midrib, whereas in the latter the veins originate in 
the midrib, and repeatedly branch and anastomose with 
each other, forming a network. The former are known 
use of the terms dorsal and ventral in place of upper and lower 
is particularly desirable in describing the surfaces of erect or 
ascending leaves, as of Iris ; or leaves in which the petiole is 
twisted so that the surfaces are either vertical, as in the phyllodia 
of Australian Acacias, or the position of the surfaces is changed, 
as in Eucalyptus. 
