ETHEREAL OILS, TERPENES 63 



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

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

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

 (Humulus lupulus), 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 camphora), 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 

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

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

 distilled off, the solid residue left is rosin. 



Most of the terpenes are colourless, highly refractive liquids, 

 which evaporate completely if sections containing them are 

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

 soluble in alcohol, chloral hydrate, glacial acetic acid, etc. Com- 

 pounds of this nature, often together with other substances, 

 commonly occur in specially differentiated cells 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 Cruciferae, cf. p. 48), and become liberated only 

 after coming in contact with the appropriate enzyme. 



The colours of flowers are due in many 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 leaves of the Mother-of -Thousands (Saxi- 

 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. 



