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fades within five minutes, and yet it flourishes unshadowed
through the whole summer. Long roots are also useful in
enabling the plants to resist the extreme changes of
temperature.
In other cases in which a plant survives through the
whole year, it is adapted to its conditions by modifica
tions which enable it to resist heat and drought, either by
economising such moisture as they acquire or by storing
it for subsequent use ; and sometimes the same modifica
tion produces both these results. With respect to the first
point many illustrations might be given. The mechanical
tissues are hardened, and very hard wood resists heat be
cause it encloses but little aqueous juices, so that there is
little to evaporate. The leaves of desert plants are small,
thereby lessening the surface of the transpiring organs; or
else they are suppressed almost or altogether. Sometimes
the inrolled margins of leaves make them assume the form
of a closed cylinder, and thus they present less surface.
The thickness of the cuticle tends powerfully to prevent
the loss of water. Many plants are protected against a
too energetic transpiration by the existence outside the
cuticle of a thick layer of a sort of varnish, without doubt
of a resinous nature. The cuticle is sometimes “ orna
mented with parallel and straight or undulating striae, or
they may be more or less reticulated.” The effect of this
modification is to temper the strong glare of the sunlight.
So, at least, it may be inferred from the fact that a sheet
of sensitive paper is not darkened to the same extent
under sheets of glass with striated and reticulated surfaces
as under a clear sheet of glass of the same thickness when
fully exposed and for the same time to sunlight.
Some desert plants secrete strong-scented ethereal oils ;
and, since Dr. Tyndall has shown how minute quantities
of such oils diffused through the air are capable of arrest