ETHEREAL OILS, TERPENES 63 



Cinnamon (the bark of young twigs of Cinnamommn zeylanicum) , 

 Ginger (the rhizome of Zingiber officinale), Pepper (the berries of 

 Piper nigrum), etc. Moreover, the active principle of the Hop 

 [Hiumilus liipiilus), which is contained in special hairs (cf. p. 105) 

 borne on the bracts of the female catkins, and that causing the 

 odour of Tea, likewise belong to the ethereal oils. 



Camphor is a solid terpene-derivative obtained from the wood 

 of the Camphor-tree [Cinnamomum campJiora), whilst turpentine 

 is a mixture of terpenes which flows from the resin-passages 

 (cf. p. 339) in the trunks of various species of Pines (especially 

 Finns pinaster) and of the Spruce Fir (Picea excelsa), when cuts 

 are made in the surface. After the oil of turpentine has been 

 chstilled off, the solid residue left is rosin. 



Most of the terpenes are colourless, highly refractive liquids, 

 which evaporate completelj' if sections containing them are 

 heated on a shde for about ten minutes. They are readily 

 soluble in alcohol, chloral h^'drate, glacial acetic acid, etc. Com- 

 pounds of this nature, often together with other substances, 

 commonly occur in speciallj' differentiated ceUs or intercellular 

 spaces within the tissues of the plant (cf. pp. 151, 152), or in 

 glandular hairs (p. 105). The ethereal oils are sometimes 

 combined with glucose, etc., in the form of glucosides {e.g. the 

 mustard oil of Cruciferie, cf. p. 48), and become hberated only 

 after coming in contact with the appropriate enz\Tne. 



The colours of flowers are due in manj' cases to pigments, 

 which are classed as anthocyanins, dissolved in the cell-sap of the 

 petals, the colour being red, blue, or violet according to the 

 acid or alkaline reaction of the sap. Such pigments are also 

 frequently present in the vegetative organs, as, for instance, in 

 the Beetroot and in the lea\'es of the Mother-of-Thousands [Sa.xi- 

 fraga sarmentosa). Their development appears to be stimulated 

 by excessive transpiration and intense illumination, conditions 

 which are realised in alpine and arctic regions where high coloura- 

 tion is a conspicuous feature. 



The yellow and red colouration of many flowers [e.g. Garden 

 Nasturtium) and fruits [e.g. Tomato) is, however, not due to 

 substances in the cell-sap, but to the presence of pigments 

 (carotin, etc.) in special plastids, termed chromoplasts. In many- 

 cases the pigment occurs in the latter in a crystalline form. 



