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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