45] CHAP. II. PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 187 



ing present in the same proportions as in water (H_,O) ; of these there are 

 the following classes : 



a. Amyloses : general formula n (C 6 H 10 O 5 ) ; of these cellulose and starch are 



the most common, the former entering largely into the composition 

 of cell-walls, the latter occurring as a reserve material in the form of 

 ' starch-grains; they are neither of them soluble in water under 

 ordinary circumstances : dextrin or amylin, a product of the action 

 of diastase on starch, is soluble in water but not crystallisable : 

 inulin (see p. 83) occurs in many Composites and all ied orders (Cam- 

 panulacese, Lobeliacese) in solution in the cell-sap; it is slightly 

 soluble in cold water and is crystallisable. The gums and mucilages 

 also belong to this group. 



b. Sucroses: C^H^On : soluble in water and crystallisable: cane-sugar 



occurs in many plants (esp. Sugar-cane and Beet-root) ; maltose is 

 the chief product of the action of diastase on starch. 



c. Glucoses: C 6 H ]2 O 6 : soluble in water and crystallisable : they occur in 



fruits (grape-sugar). 



The sucroses and glucoses are commonly known as sugars. 



A substance termed Mannite (C 6 H 14 O 6 ) occurs in the cell-sap of Fraxinus 

 Ornus and some other plants: though not a carbohydrate, it is closely 

 allied to this group ; crystallisable, but not readily soluble in water. 



2. Organic Acids: these occur in the plant either free or, more commonly, 

 as neutral or acid salts in combination with organic or mineral bases ; 

 some are constituents of the fats and fixed oils (e.g. palmitic and oleic 

 acids ; see below) : the more common are oxalic acid (H 2 C 2 O 4 ) malic acid 

 (H 2 C 4 H 4 O 5 ), tartaric acid (H 2 C 4 H 4 O 6 ), citric acid (H 3 C 6 H 5 O 7 ). 



3. Glucosides : substances of complex constitution which owe their name 

 to the fact that they give rise, on decomposition, to glucose among other 

 products : such are amyrjdalin, C 2> H 27 NO n (seeds, etc., of many Rosacese) ; 

 coniferin, Ci 6 H 22 O 8 (coniferous wood) ; myrosin, or myronate of potash, 

 KCjoH-sNS^o (seeds of Mustard); salicin, C 12 H 18 O 7 (in bark of Willows 

 and Poplars) ; yallo-fannin, C 34 H 28 O 22 (in Oak-bark). 



Though some of these substances (e.g. amygdalin and myrosin) contain 

 nitrogen, it is more convenient to classify them with the more numerous 

 non-nitrogenous glucosides. 



4. Fats and Fixed Oils : these substances, as they occur in the seeds and 

 fruits of plant?, are mixtures of free fatty acids with glycerin-compounds 

 (glycerides) of fatty acids ; thus palm-oil is a mixture of palmitic and oleic 

 acids with their glycerides palmitin, C 3 H 5 (Ci 6 H 31 O) 3 O 3 , which is a solid fat, 

 and olein, C 3 H 3 (C 18 H 33 O) C O 3 , which is a fluid fat or oil : olive-oil consists 

 chiefly of olein with some palmitin : castor-oil, of ricinolein (the glyceride of 

 ricinoleic acid) and stearin (the glyceride of stearic acid) : linseed-oil, of 

 linolein (the glyceride of linoleic acid) and palmitin. 



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- 



