1879J Leaves and their Functions » 407 
than by their forms that we know they are leaves. Under- 
ground stems or rhizomas have them at each point or node 
as little thin scales. Buds are enveloped in peculiar cere- 
ments, which generally fall away soon after the ordinary 
leaves have begun to expand ; those enveloping scales are 
only leaves in a modified form. They are quite prominent 
in the hickory and horse-chestnut. The scales of bulbs, as 
of the lily, are simply modified leaves. Flowers are only 
aggregations of metamorphosed leaves. But it is with leaves 
as foliage that we are more immediately concerned at pre- 
sent. A complete leaf consists of three parts — the stalk or 
stem (petiole) on which it rises, the expanded blade or 
lamina, and two small leaf-like appendages at the base of 
the leaf-stalk called stipules. The only essential part is the 
blade, as this may be sessile on the stem without petiole or 
stipules. The blade of a leaf consists of three portions : — 
the woody framework, ribs, or veins ; the green cellular por- 
tion, pulp ; and the outside covering, or epidermis. The 
epidermis, which is really an extension of the outer bark of 
the stem, is composed of closely united, transparent cells, 
with frequent openings through it called stomata or 
breathing-pores. These vary in number from 800 to 170,000 
to the square inch of surface. It is through these that 
water is exhaled from the plant. They are more numerous 
in the leaves of plants growing in moist situations and sur- 
rounded by a damp atmosphere. The pores dilate with the 
increase of humidity and contract with the increase of 
aridity. Plants growing in arid climates have but few 
stomata, and these are very small. While the most of 
foliage appears to be made on the principle of exposing the 
greatest possible surface to the air, some forms of vegetation 
seem to be constructed for the accomplishment of the very 
reverse of this. Thus the various species of Cactus , whose 
native habitat is the hot, arid plains of the South-west, are 
constructed on the principle of presenting the least extent 
of surface to the air, and this surface is covered with an 
epidermis that is almost impervious to water. This is ne- 
cessary to prevent excessive transpiration in that very dry 
climate. The pulp or parenchyma of the leaf is made up 
of several layers of cells. These cells are small globular 
sacs, varying from i-i200th to i-25oth of an inch in dia- 
meter. A layer of these, of a rather elongated form, is 
arranged immediately beneath the epidermis of the upper 
side of the leaf with the ends to the surface. These are 
crowded quite closely together. Another layer, not quite so 
much elongated and less compadly arranged, is found on 
