408 Leaves and their Function [June, 
the under side of the leaf. Between these two layers are 
numerous globular cells that seem thrown together without 
any great regularity or order. Among these are numerous 
irregular passages, intercellular spaces, through which water 
and air circulate. These reach the surface through the 
stomata of the epidermis. It is worthy of notice that by 
far the larger part of these breathing pores are on the under 
surface, and this surface always seems to avoid direCt sun- 
shine. If a leaf is inverted, turning the bottom side 
upward, it will, if possible, return to its natural position, 
and if prevented from so doing it will soon die. A few 
leaves have been known to grow in a vertical instead of a 
horizontal position. The framework of leaves consists of 
wood, and is intended to give firmness and support to the 
leaf. It is divided into numerous veins or nerves that ramify 
every part of the green parenchyma. There are two dis- 
tinct systems of venation of leaves — the parallel-veined and 
the net-veined. In the former the fibres run nearly parallel 
from one extremity of the leaf to the other ; such leaves are 
usually long and narrow, linear, as in the grasses, corn, &c. 
In the other the veins are netted, ramifying the leaf in all 
directions, and dividing the parenchyma into numerous 
small squares and diamonds. This style of venation exists 
under two forms : in one a principal vein, midrib, extends 
from the base to the apex of the leaf, and from this nu- 
merous smaller veins branch off and run to the margin ; in 
the other there are three or five nearly equal ribs running 
the length of the leaf. The first is feather-veined, from its 
resemblance to a feather ; and the other is palmately veined, 
the main ribs branching out like the fingers of a hand. 
The shape of a leaf is generally determined by the manner 
of its venation. The two principal styles of venation be- 
long to and denote two different classes of plants, the 
parallel-veined belonging to the monocotyledonous, and the 
net-veined to the dicotyledonous divisions of the vegetable 
kingdom. Thus the veining of a small portion of a leaf 
will indicate to which of these classes the plant upon whifch 
it grew belonged. 
The green colour of leaves comes from a granular sub- 
stance, chlorophyll, found in the cells of the parenchyma. 
In its absence no true vegetable structure can be built up 
from the original elements, and it can operate only in the 
presence of sunlight. Low cryptogamic plants will grow in 
the dark, but they contain no proper chlorophyll. Chloro- 
phyll has been found to be composed of two different sub- 
stances, — xanthophyll, a yellow substance, and cyanophyll, 
