BY SELF-ADAPTATION TO THE ENVIRONMENT, 255 
grow outside Cairo, but not quite on the desert. They never 
receive any water by artificial irrigation whatever, yet are in a 
very flourishing condition. 
Halophytic plants, and others yielding ethereal oils, though 
not uncommon in the Egyptian deserts, in consequence of the 
general presence of salts in the sand, are of course especially 
characteristic of more saline areas, as of the Asiatic steppes. 
These extensive regions agree with the more Southern deserts in 
excessive aridity and heat; and consequently we find the same 
characteristic features in the plants of both alike, such as the 
production of spines, hairy epidermis, saline sap, and the secre- 
tion of ethereal oils. The Chenopodiacee are especially cha- 
racteristic of both regions, the salts of soda imbibed by the roots 
and retained within the plant rendering the water less easy of 
evaporation. 
Similarly there is a great secretion of ethereal oil in conse- 
quence of intense heat. Thus Grisebach observes that Arabia 
is on this account distinguished by its aromatic and resinous 
plants, of which several are equally indigenous in the steppes of 
the East *. While, however, these environmental conditions are 
the direct causes of the secretion of the oils, these latter, in their 
turn, aid in checking the loss of water. Thus Grisebach ob- 
serves :—“ Les huiles éthérées paraissent également agir d’une 
maniere restrictive & ?égard de la dépense de la vapeur aqueuse, 
lorsque les organes de végétation sont richement pourvus de ces 
éléments aromatiques. L’buile s’évapore plus facilement que 
Veau, et entoure chaque feuille d’une atmosphére imprégnée de 
vapeurs odoriférantes. On sait que les vapeurs de substances 
différentes restent indépendantes les unes des autres dans un 
espace qui en est saturé, mais il n’en est pas de méme lorsqu’elles 
sont dégagées avec rapidité des liquides, dans des conditions 
ou il ne peut étre question de saturation. Sans doute, cette 
rapidité est retardée en présence d’une autre vapeur susceptible 
de se produire plus aisément. C'est I’huile éthérée seule que la 
walled, but provided with numerous small transverse slits. They suggest the 
idea that they are means of temporarily storing water until it is required to be 
transinitted elsewhere; and would seem to support M. Vesque’s view of the 
use of vessels, or rather tracheides, as ‘‘ réservoirs vasiformes.” Op. cit. p. 38 
[supra, p. 227]. 
* Vég. du Globe, ii. p. 129. 
