415 
ing radiant heat, it has been suggested that this is one of 
the many resources to which desert plants appeal in order 
to reduce the ill effects of the heated atmosphere which 
surrounds them. Sometimes desert plants effect a storage 
of water ; this is the case with succulent leaves. The 
increased substance of the leaf is accompanied by a greater 
development of palisade-tissue, with diminished intercel 
lular passages. Certain mineral salts are secreted by some 
plants. Later on, after the rainy season is over, these 
excessively hygrométrie salts absorb dew, which is thus 
transmitted to the plant, and this enables it to retain its 
bright green character all through the hot season. Some 
desert plants have leaves of intense hairiness. This fea 
ture of desert plants is an invaluable means of lessening 
the heat, by forming a non-conducting surface. It is also 
a means of absorbing dew during the summer when no 
rain falls. Sometimes the same hairs answer first one 
purpose and then another ; they are “ bladdery and filled 
with water.” These latter may finally collapse, dry up, 
and form a glassy sheet. 
There can be no reasonable doubt that these modifica 
tions are useful to the plants. It may be asked,—How 
have they been acquired ? The spinescent leaves which 
protect the plant, the long root which finds its way to the 
water and supplies the plant with an inexhaustible supply, 
the buds which endure when the rest of the plant is des 
troyed, the woody texture which protects the water stored 
in the scales of the bulb, the hardened wood, the small 
leaf, the enfolded leaf, the thickness of the cuticle, the 
coat of varnish, the striated surfaces, the ethereal oils, 
the succulent leaves, the palisade-tissue, the mineral salts, 
the “ hairiness ” of the leaf,—all are believed to result 
from the direct action of the outward conditions upon a 
responsive organism. These modifications are made “ by