240 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
countenance from the fact that just those desert plants which do 
develop much parenchyma and become exceedingly fleshy, as the 
Aloinee, Euphorbia, Cactacee, Zygophyllum, «c., are generally 
entirely hairless. 
Intense hairiness is certainly one of the most conspicuous 
features of desert plants, and is an invaluable means of lessening 
the heat by forming a non-conducting surface ; and, on the other 
hand , is ameans of absorbing dew during the summer when no 
rain falls. 
Dr. G. Volkens remarks that a multitude of cases establish 
the empirical deduction that drought is correlated with the pre- 
sence of much hair, though what the real causal connection may 
be is not so clear tohim*. He observed that as transpiration tends 
to increase, so does the relative quantity of hair, till (it may be 
added) the quantity becomes so great as to check the very process 
which may have had something to do with bringing it about. 
In desert plants the hairs are of different forms. They may 
be stiff, straight, and adpressed to the surface, all lying in one 
direction ; or the “needles ’’ are interlacing ; or the hairs may be 
of a twisted cottony character and cover the surface with a layer 
of wool; or, again, they may be stellate and flat, the branches 
interlacing so as to produce a dense coating of felt; or they may 
be bladdery and filled with water. These latter may finally 
collapse, dry up, and form a glassy sheet. 
There are also peculiarities in the structure of the hairs them- 
selves which are remarkable. In many cases the cavity is quite 
filled up with the exception of the broad basal part of the cell, 
while the outer surface may be densely coated with wax, either 
entirely or with “ gashes” and “ pores ” (Diplotaxis Harra), or 
else the basal portion is quite devoid of it (Heliotropium luteum), 
so that water can readily pass from without into the interior. 
Dr. Volkens observes, with regard to the filling up of the lumen 
with cellulose matter, that this is connected with the swelling up 
of the inner membrane, and shows (by treatment with suitable 
reagents) that it is a substance which, when water is absorbed, 
can hold it fast with great strength. This, of course, greatly 
retards transpiration. 
I have found in some cases, besides the closely applied felt, 
that there are taller branching hairs standing much above the 
* He makes no allusion to M. Mer’s observations, which were probably there- 
fore unknown to him, 
