NUTRITIVE ORGANS OF PLANTS. 117 
by the blade of the leaf. Some leaves are thick and fleshy. The forms 
of leaves are very various, and many are divided into lobes or segments. 
A leaf much divided into lobes may yet consist altogether of one piece ; 
and a leaf which consists of one piece, whether lobed or not, is called a 
simple leaf. Many leaves, however, are compownd—that is, they are made: 
of a number of pieces, the leaf-stalk branching by joints, and sometimes 
branching again and again, each branch or branchlet bearing a separate 
blade, called a leaflet. Such leaves are called pinnated,! when the leaflets 
are arranged on opposite sides of the stalk, as in the ash or the laburnum. 
A leaf may be bipinnate—that is, twice pinnate—or the leaf-stalk may be 
still more subdivided. In some compound leaves, the leaf-stalk divides 
' into three branches, as in the clover or trefoil ; in others it sends off a 
number of leaflets in a radiating manner from its extremity, as in the 
horse-chestnut. 
Stomata.—In the epidermis of leaves there are minute openings, called 
stomata, a Greek word signifying mouths. These are also found in the 
epidermis of young shoots, and of other green parts of plants. They serve 
for the admission of air as the plant requires it, and for the exhalation of 
what it gives off into the air; and leaves have therefore sometimes been 
called the Jungs of plants. The stomata are very small, and can only be 
discerned by the aid of a microscope. There are sometimes 160,000 or 
more in a square inch of surface ; and in those plants which have fewest 
and largest stomata, there are about 200 in a square inch. 
Circulation of Sap.—Plants have no organ resembling the heart of an 
animal, and the circulation of sap which takes place in them is of a very 
different nature from the circulation of blood. The sap imbibed by the 
roots ascends through the vessels of the stem, and passes from vessel to 
vessel by osmotic action. In plants wholly formed of cellular tissue, the 
Sap seems to proceed in any direction from one cell to another, till the 
whole substance of the plant is permeated ; in vascular plants, particular 
parts of the plant appear to be chiefly concerned in this process, which, 
however, is very imperfectly understood. It is known that the 
sap circulates most abundantly through the youngest layers of wood, 
whilst in the old and thoroughly hardened layers it almost entirely ceases, 
- and these may therefore almost be regarded as having ceased to live, and 
as useful to the plant only by giving strength to its stem. The sap reaches 
every part of the plant, being conveyed through the finest leaf-stalks and 
flower-stalks, and penetrating the most delicate parts of the flowers. In 
the leaves and other green parts, it is modified by the action of the air 
and light, and afterwards descends again to the root, which it is generally 
believed to do through vessels in the bark, giving off supplies of nourishment 
1 From Latin pinna, a wing. 
