BY SELF-ADAPTATION TO THE ENVIRONMENT. 237 
are common in desert plants. M. Vesque offers the following 
interpretation * :—“ Tl serait bien difficile de déterminer le role 
de ces dessins cuticulaires par l’expérience; mais étant donnée 
cette circonstance singuli¢re que les épidermes plans en sont 
ordinairement dépourvus, tandis que les parties convexes en 
présentent presque toujours, il est permis d’émettre une hypo- 
thése & mes yeux fort plausible. Chaque cellule convexe repré- 
sente en effet une lentille convergente qui, malgré ses faibles 
dimensions peut, surtout dans les pays chauds, notablement 
surélever la température en un point déterminé de la cellule 
épidermique ; il est donc important, dans ce cas, de remplacer la 
vitre lisse par une vitre cannelée qui a pour “effet de disperser, 
d’égaliser la lumiére incidente; de cette mani¢re on comprend 
pourquoi, dans un grand nombre de cas, les cellules convexes 
qui avoisinent les stomates ou les poils enfoncés au-dessous du 
niveau de |’épiderme et celles qui se relévent en petites sallies 
autour de la base des poils sont striées tandis que les autres ne 
le sont pas.” 
As far as sheets of glass with striated and reticulated surfaces 
can imitate a cuticle, I find that a sheet of sensitive paper is 
not darkened to the same extent under them as under a clear 
sheet of glass of the same thickness when fully exposed, and 
for the same time, to sunlight; though nothing could be 
deduced from any differences of temperature under the same 
circumstances. 
Priostsm.—Since a more or less excessive hairiness is a cha- 
racteristic feature of the great majority of plants growing in hot 
and barren deserts, the question arises as to what is the cause. 
Now any extra outgrowth, even if it be but epidermal tri- 
chomes, implies the presence of more nutritive materiais at the 
disposal of the plant at the spot than when they are not formed 
at all. M. Mer, who studied the question, came to the con- 
clusion that, ceteris paribus, hairs are due to a localized extra 
nourishment, and therefore frequently occur upon the ribs and 
veins, 7. e. immediately over the channels of sap. He thinks 
this view is supported by such a case, e. g., as Rhus Cotinus, 
in which the abortive pedicels, which bear no fruit, develop a 
Jarge amount of hairs, while the pedicels which bear fruit have 
few or none. The excess of hair is therefore presumably due 
to a compensatory distribution of sap. 
* Op. cit. p. 34 [supra, p. 235, note]. 
