417 
surface of leaves is dependent ultimately upon the in 
creased amount of light (and heat ?) which appears to 
deoxidise chlorophyll and to leave wax as one of its 
products. The striated surfaces are associated with cells 
which are convex towards the exterior, and M. Vesque 
explains the correlation by supposing that the striated 
surfaces tend to lessen the effect of the sun’s rays 
which would otherwise be intensified by the form of the 
cells, which would act as lenses. In this case the utility of 
the correlation is obvious, and, if we may argue from the 
analogy presented by other cases, it may be regarded as 
the result of the action of the environment, and, probably 
of correlated variation also. 
The secretion of ethereal oil is the consequence 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. Succulent leaves are the result of 
the direct action of salt. The presence of salts has been 
proved by M. Lesage to be the immediate cause of suc- 
culency in maritime plants of temperate climates, and he 
succeeded in making plants succulent which are not so 
ordinarily. M. Battandier cultivated Sedum Clusianum, 
and the leaves at once began to assume a flatter character. 
He also observes that Sedum rubens and Sedum Mag?ioli 
have flat leaves in a wet season, but cylindrical leaves in 
a dry one. The common maritime Samphire of temperate 
climates, when cultivated in a garden, became luxuriant 
and bore flat and smooth leaves. 
The production of hair occurs when normal development 
is arrested. This explanation will satisfactorily explain 
the existence of the hairiness of plants in the deserts; for 
drought, aided by the barrenness of the soil, tends to 
arrest the development of parenchymatous tissues. These 
AA