254 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
Africa and Beluchistan; Pulicaria arabica has a particularly 
powerful odour. Since Dr. Tyndall has shown how minute 
quantities of such oils diffused through the air are capable of 
arresting 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 ; 
and just as the presence and quantity of opium, hasheesh, 
aconitine, &c. secreted by plants varies greatly with the climate, 
so it is reasonable, in the absence of strict investigations, to 
assume that these oils are in an excess through the intense heat 
and other conditions of the climate of deserts. 
Another of the more interesting secretions may be here 
mentioned, viz., of certain mineral salts, which by their hy- 
grometric properties enable the plants to absorb dew from the 
air during the hotter months and thus transmit it to the tissues 
within. Reauwmuria hirtella, species of Tamarix, Frankenia 
pulverulenta, species of Statice * and Cressa are the more remark- 
able instances. The first-named plant having developed its new 
foliage in early spring, when water is comparatively copious, the 
leaves in the early mornings appear covered with dew-like drops, 
no doubt due to root-pressure. As the sun mounts the water 
evaporates, and the plant is now covered with a dust-like secre- 
tion of chlorides of sodium, of calcium, and of magnesium; the 
two latter being of less proportion than the first. There are 
special, two-celled glands which secrete these salts. Later on, 
after the rainy period is over, these excessively hygrometric salts 
absorb dew, which is transmitted to the plant, and thus enable it 
tu retain its bright green character all through the hot season f. 
In a similar manner a large number of very lofty Tamarix trees 
* See Mr. J. Wilson’s paper on “ Mucilage and other Glands of the Plumba- 
ginee,” Ann. of Bot. iv. 1890, p. 231. 
+ I had a curious experience with this plant when drying it in w press for 
my herbarium. Placing freshly gathered specimens in the usual way between 
drying-papers, I proceeded to change them after three days. To my surprise 
I found the sheets perfectly saturated where the specimens were lying. They 
themselves were covered with dew-like drops, although under strong pressure. 
The salts had in fact rapidly drawn out the moisture from within the plant. 
After fresh papers were supplied the plant dried quickly. 
There are some peculiarities in the anatomy of the leaf of Reawmuria which 
Dr. Volkens does not allude to, though he has figured the two-celled salt- 
glands, &c. One is the remarkable forms the “ tracheides” of the leaves assume. 
Instead of being more or less straight tubes, they bulge into bag-like processes 
of three or four sides, or else assume various irregular shapes. They are thick- 
