we generally call leaves are in
reality vertically-flattened leaf stalks.

In other cases the stem itself is green, and to some extent replaces the
leaves. In our common Broom we see an approach to this, and the same
feature is more marked in Cactus. Or the leaves become fleshy, thus
offering, in proportion to their volume, a smaller surface for
evaporation. Of this the Stonecrops, Mesembryanthemum, etc., are
familiar instances. Other modes of checking transpiration and thus
adapting plants to dry situations are by the development of hairs, by
the formation of chalky excretions, by the sap becoming saline or
viscid, by the leaf becoming more or less rolled up, or protected by a
covering of varnish.

Our English trees are for the most part deciduous. Leaves would be
comparatively useless in winter when growth is stopped by the cold;
moreover, they would hold the snow, and thus cause the boughs to be
broken down. Hence perhaps the glossiness of Evergreen leaves, as, for
instance, of the Holly, from which the snow slips off. In warmer
climates trees tend to retain their leaves, and some species which are
deciduous in the north become evergreen, or nearly so, in the south of
Europe. Evergreen leaves are as a rule tougher and thicker than those
which drop off in autumn; they require more protection from the weather.
But some evergreen leaves are much longer lived than others; those of
the Evergreen Oak do not survive a second year, those of the Scotch Pine
live for three, of the Spruce Fir, Yew, etc., for eight or ten, of the
Pinsapo even eighteen. As a general rule the Conifers with short leaves
keep them on for several years, those with long ones for fewer, the
length of the leaf being somewhat in the inverse ratio to the length of
its life; but this is not an invariable criterion, as other
circumstances also have to be taken into consideration.

Leaves with strong scent, aromatic taste, or acrid juice, are
characteristic of dry regions, where they run especial danger of being
eaten, and where they are thus more or less effectively protected.

ON HAIRS

The hairs of plants are useful in various ways. In some cases (1) they
keep off superfluous moisture; in others (2) they prevent too rapid
evaporation; in some (3) they serve as a protection against too glaring
light; in some (4) they protect the plant from browsing quadrupeds; in
others (5) from being eaten by insects; or, (6) serve as a quickset
hedge to prevent a