28 A MANUAL Of THE CONIFEKJI. 



Ferns, are polymorphous,* a principle that manifests itself throughout 



the order, hut is much more common in some tribes than in others ; 



it is less frequent in Pinus than in Abies, very usual in the Yew, 



and most common in the Cypress tribe (Cupressus, Thuia, Eetinospora, 



Juniperus, &c). It is most observable in plants in their young state, 



but when the departure from the usual type is not very great, the 



difference gradually disappears as the plant becomes older. 



Secretions. — The Secbetions are abundant, and flow copiously from 

 wounds made in the stems of the trees during the growing season, 

 especially in the Fir and Pine tribe ; they are known as Turpentine, 

 Kesin, Tar, &c, which are much used in the. arts, chiefly as preserva- 

 tives against decay. " The turpentine is generally contained in special 

 receptacles in the substance of the wood, but sometimes it collects 

 in blisters underneath the bark, which appear during the strong 

 heats of summer. It flows from these as a liquid juice, which thickens 

 on exposure to the atmosphere, when incisions are made in the 

 stem."f The crude turpentine thus obtained consists of a volatile 

 or essential oil and resin, which can be separated by distillation. 

 Oil of turpentine in its pure state is a colourless liquid of powerful 

 odour, % almost insoluble in water, but dissolving in alcohol or 

 ether, and absorbing oxygen rapidly from the air, especially when 

 mixed with ceruse or white lead ; it dissolves sulphur, and is a 

 good solvent for grease. § It is also a powerful solvent for resins, 

 which form the bases of most varnishes, and from its great vola- 

 tility, it quickly flies off or dries away, leaving a thin coat of the 

 varnishing substance on the surface on which it has been applied. || 

 It is the only known volatile oil that mixes readily with paint 

 without affecting its essential properties, diluting it that it may 

 flow freely from the painter's brush, and by its volatility causing 

 the paint to dry rapidly. Tar is impure Turpentine, altered by the 

 heat used to separate it from the wood. Pitch is simply Tar 



* This word explains nothing, it simply expresses a phenomenon, the cause of which 

 lies deeper than physiological science has yet reached. 



+ Dr. Carpenter, Vegetable, Physiology, p. 213. 



X Chemically a hydro-carbon, having for its formula C"> H16, specific gravity 86 

 (when pure distilled water at 15° C. is 1), it toils at 160° G. -Chemistry, by Dr. Williams, 

 m Clarendon Press Series, § 348. ' "uuamsj 



§ Idem. 



II Dr. Carpenter, Vegetable Physiology, p. 213;, 



