Function of the Ethereal Oils of Xerophytic Plants. 319 



thought that they had found an excellent explanation of the very 

 common and general occurrence of aromatic plants in dry climates. 

 It is a well-known fact that many plants of desert regions are 

 strongly scented, agreeably or not with regard to our own olfactory 

 organs as the case may be. It was thought that these plants, 

 by surrounding themselves with an atmosphere of aromatic vapour, 

 were able to guard off a portion of the heat of the sun's rays and 

 that in this way their transpiration would be kept down con- 

 siderably. 



It must not be thought that the expression of this view is of 

 isolated occurrence. On the contrary, several of our foremost 

 biologists and phyto-geographers have accepted it. Haberlandt, in 

 his "Physiological Anatomy of Plants," looks upon Tyndall's 

 experiments as quite conclusive ; Volkens, Drude, Warming, and 

 others share his views, and MacMillan,* in " Minnesota Plant- 

 life," adopts the same, as the following passage will show: " Many 

 desert plants we know by their pungent odour, e.g., wormwoods and 

 sage-bushes. It has been shown that the vapour of the ethereal oils 

 •existing in such plants, when commingled with the atmosphere, 

 reduces its permeability to heat, and thus the constant exhalation 

 of perfume from the body of a wormwood is to be regarded as a 

 device for tempering the heat of the sun." 



On the other hand I must not omit to state that, as far as I have 

 been able to ascertain, Tyndall himself never interpreted his results 

 in this way, and that consequently the designation of this hypothesis 

 as " Tyndall's theory " places the responsibility on the wrong 

 person. 



Some authors, however, did not accept this explanation, although, 

 of course, it would have been very difficult to disprove it. Eecently, 

 DettOjf a pupil of Stahl, has repeated the experiments with snails 

 and slugs in various ways, and has arrived at the conclusion that 

 the function of these oils of exogenous or superficial origin is the 

 same as that of the internally stored oils, i.e., that they are means of 

 protection of the plants against injuries by animals. "I do not 

 think that this property of essential oils (viz., their diathermancy) 

 is of greater importance to the life of the plant as, e.g., their colour, 

 their density, or their action on the polarised light." 



I may say that I have arrived at the same conclusion, principally 

 on account of observations made during the last twenty years in 

 various parts of South Africa. 



* MacMillan, " Minnesota Plant-life," Report of the Survey, 1899, p. 467. 

 f Detto, Carl, "Ueber die Bedeutung der iitherischen Oele bei Xerophyten," 

 Miinchen, 1903. 



