CHAPTER II. SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 709 



plants (flowers, fruit, etc.), and it is to thein that the odours of plants are 

 chiefly due. They usually consist of a mixture of oxygenated compounds with 

 hydrocarbons of either the formula C^jHu. (terpenes) or C15H24 : many of the 

 oxygenated compounds are solids held in solution in the liquid hydrocarbon ; 

 thus, turpentine (from Conifers) is a solution of reifin (or abietic anhydride, 

 C44He204) in the terpene called terebene ; camphor-oil consists of camphor 

 (CjoHnjO), a solid substance, dissolved in a terpene ; oil of cloves consists 

 chiefly of eugenol (CjoHi^Oa) with a hydrocarbon C15H24. Some essential oils 

 consist simply of a single substance the composition of which is various ; thus, 

 oil of bitter almonds is an oxygenated substance, C7HgO (benzoic aldehyde); 

 oil of mustard is a sulphur-containing nitrogenous substance, CsHs.NCS (allylic 

 isothiocyanate). 



Allied to these hydrocarbons are the solid substances caoutchouc and gutta- 

 percha which occur in milky latex : they have the composition ^(CsHg). 



The organic compounds can for the most part be resolved into 

 volatile products — chiefly carbonic acid, water, and nitrogen — by 

 exposure to great heat with free access of air, that is, by combus- 

 tion. The inorganic residue is a white, or, if the combustion is 

 imperfect, a grey powder, the ash. 



As the result of chemical processes attending the combustion, 

 the sulphur and phosphorus previously contained in the organic 

 compounds appear as sulphates and phosphates in the ash, and the 

 carbonic acid formed during combustion combines with some of 

 the inorganic substances. These, therefore, must not be included 

 in an accurate estimate of the constituents of the ash. 



The ash usually constitutes but a small percentage of the whole 

 dry solid of the plant. The amount of ash increases with the age 

 of the plant, or of any part of it, inasmuch as there is no appreci- 

 able excretion by the plant of the mineral substances absorbed. 

 The percentage of ash in the dry solid of the plant, or of any 

 organ, may vary widely at different times. The following analyses 

 of various portions of plants will give an idea of its amount and 

 composition : — 



1000 PAETS OF DRY SOLID MATTER CONTAIN : 



