THE PBOTOPLAST AS HOUSE-BUILDER 



65 



Photo by] 



Fia. 94. BIRCH (Betula alba). 



ihowing the delicacy of the twigs, the light character of the foliage, and the short cylindrical cones. 

 EUROPE, N. ASIA. 



[E. Step. 



ment, during the process of tanning, of a volatile oil obtained by the 

 distillation of Birch bark (Betula). The oil has a brown or black colour, 

 and a little of it poured on paper and allowed to dry gives to the paper 

 the scent peculiar to russian leather. On a future occasion, when the 

 odours of flowers in relation to insects will be our subject, allusion will be 

 made to Kerner's helpful classification of the aromatic oils, and some 

 further light will be thrown on this very interesting subject. 



Professor Tyndall found that infinitesimal quantities of these essential 

 oils thrown off into the air enormously increased its power of absorbing 

 heat-rays of low tension ; and Dr. George Henderson, F.L.S.,* has suggested 

 that in this way these oils may often prevent injury from frost at one of the 

 most critical periods of a plant's life, namely, when it is setting its fruit. He 

 says, " In the low hills of the Punjab Himalaya, from 1,000 to 4,000 feet 

 above the sea and 10 to 20 miles across, in the end of March and in April, 

 when most of the plants are coming into flower, the blossoms are apt to be 

 blighted by late frosts, at least one would expect this ; but at that season 

 the air is filled with the odours of essential oils from these blossoms to such 

 an extent as to be at times (and especially on a still night, when frost most 

 often occurs) quite overpowering. My theory is that these essential oils 



* Proceedings of Linnean Society, 1903. 

 8 



