320 Transactions of the South African Philosophical Society. 



If the production of a diathermanic atmosphere around the plants 

 would be the principal function of the ethereal oils, one should 

 expect that the intensity of secretion would be largest during the 

 hottest part of the day and the driest season of the year. That is, 

 however, not the case in South Africa, neither in the Karroo nor in 

 the South- Western region. On the contrary, a considerable number 

 of plants, although very aromatic, emit hardly any scent under 

 these conditions, but are bathed in odour during damp or foggy 

 weather, even when dripping wet. 



This behaviour of plants of both groups, i.e., such with internal as 

 well as with external organs of secretion, is, I think, fatal to the 

 "diathermanic" theory, for how could we assume that substances 

 which are produced by specially constructed and highly diversified 

 organs should be so universally destined to be wasted ! 



The following plants have been observed by me to exhale their 

 scent when the atmosphere is moist, but do not do so during dry 

 weather : — 



Coleonema album B. et W., Diosma vulgaris Schlecht., and many 

 species of Agathosma — in fact, most Eutaceae. The scent of Coleonema 

 in misty weather is so strong that one notices its neighbourhood 

 even yards away from it. Haberlandt has shown that the oil 

 cavities of some plants of this order possess a specially constructed 

 lid, which is thrown off, thus allowing the oil to escape, when the 

 leaf is strained in any way, e.g., when touched by an animal. 

 I have noticed that the turgor of the leaf due to a plentiful supply 

 of moisture at a time when there is no transpiration produces to 

 some extent the same effect. 



Another common plant of our hills and mountains behaves in a 

 similar way — that is Bubon Galbanum L., the wild celery, also called 

 the blistering bush. The leaves contain a highly volatile essential 

 oil, but there is usually no scent about the bush, and our moun- 

 taineers did not know for many years that this was the culprit 

 which produced the blisters on their hands, for the effect shows 

 itself only a day or two after one has touched the bush. The 

 virulence varies considerably, for sometimes one may handle a 

 bunch of the plant unpunished, and at others a slight touch will 

 do the blistering. The explanation is simple when once known. 

 In dry weather one has to crush the leaves in order to obtain any 

 effect, and even then I have sometimes failed when experimenting 

 with it, but in damp weather the oil vessels burst at the slightest 

 touch or movement of the bush, hence its fairly strong smell at 

 such a time. 



Even the common rhenoster bush, Elytrojmppus rhinocerotis 



