362 CHANGES OF ORGANIC MATTER IN THE PLANT. 



the formation of this substaDce : Many authors regard it as a 

 product of tlie retrograde metamorphosis of certain carboh}-- 

 drates ; Sachsse thinlis that it alvva\-s attends the formation 

 of cellulose from starch, and that there is a slight evolution 

 of carbonic acid ; Wiesner regards it as intermediate in the 

 series which begins with the carboh3-drates and ends with the 

 resins. This last view is also held by Hlasiwetz, who has ob- 

 tained the same products from tanniu as from the resins, when 

 each was fused with potassic hydrate. 



It is a significant fact that all the barks which are rich in tannin 

 are also rich in starch. 



Nothing is positively known as to the function of tannin and 

 its associated bodies in the plant. B}' Hartig they have been 

 looked upon as reserve materials ; but Schroeder was not able 

 to verify Hartig's observations. Bj' most observers these sub- 

 stances are regarded as waste products, having no further nutri- 

 tive function, but possibly playing some part in the formation of 

 colors. The following table ^ shows their amount in some of the 

 barks and other parts used in tanning : — 



Galls 30-77 per cent. 



Catechu 40-50 



Divi-Divi 30-40 " 



Sumach 12-18 



Oak bark 7-20 



Willow bark 8-12 



Hemlock bark 13-16 



954. Products free from nitrogen. V. Most GIncosides. These 

 are substances which under certain conditions, especially by the 

 action of unorganized ferments, are broken up into glucose or 

 some allied sugar, and at the same time some other body capable 

 of further decomposition. Most of them are soluble in water. 

 The following are among some of the best known : salicin, coni- 

 ferin, aesculin, quercitrin. Tannin is often placed among the 

 Glucosides. 



955. Products free from nitrogen. VI. Ethereal oils. These 

 are volatile liquids generally approaching Terpene (CuiH,,,) in 

 chemical composition. Nothing is certainly known as to their 

 formation in the plant. Thej' are not again taken up as plastic 

 matter, but simply serve some function, often that of attraction 



1 For other determinations see Ebermayer's Chemie der Pflanzen, p. 452, 

 from which most of the above are taken ; also see the excellent table in the 

 Tenth Census, vol. ix., p. 265. 



