708 PART IV. THE PHYSIOLOGY OF PLANTS. 



Some colouring-matter* are also nitrogenous (e.g. chlorophyll, and indigo 

 C 8 H 5 NO), as also some glucosides (see below). 

 The principal non-nitroyenuu* substances are : 



1. Carbohydrates : substances consisting of C, H. and 0, the H and O being 

 present in the same proportions as in water (H 2 0) ; of these there are the 

 following classes : 



a. Amyloses : general formula n (C 6 H 10 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 circum- 

 stances : dextrin or amylin, a product of the action of diastase on starch, 

 is soluble in water but not crystallisable : inuliii occurs in many Com- 

 posite and allied orders (Campanulaceae, Lobeliaceas) 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 12 H 22 U : soluble in water and crystallisable : cane-sugar 



occurs in many plants (esp. Sugar-cane and Beetroot) ; maltose is the 

 chief product of the action of diastase on starch. 



c. Glucoses : C 6 H 12 O 6 : soluble in water aud crystallisable : they occur in 



fruits (grape-sugar). 



The sucroses and glucoses are commonly known as sugars. 



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

 Oi-i'Us and some other plants : though not a carbohydrate, it is closely allied to 

 this group ; crystallisable, but not readily soluble in water. The glucoses are 

 derivatives (aldehydes) of mannite (an alcohol). 



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 4 ), malic acid (H 2 C 4 H 4 5 ), 

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



3. Glucusides : substances of complex constitution which owe their name to 

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

 such are amygdalin, C 2 oH 27 NO u (seeds, etc., of many Rosace) ; conifer in, 

 C 16 H 22 8 (coniferous wood) ; my rosin, or myronate of potash, KC 10 H 18 NS 2 10 

 (seeds of Mustard) ; salicin, Ci 2 H 18 7 (in bark of Willows and Poplars) ; yallu- 

 tannin, C 34 H 28 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 Oil* : these substances, as they occur in the seeds and 

 fruits of plants, are mixture's of free fatty acid* with glycerin-compounds 

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

 acids with their glyceridea palmitin, C 3 H 5 (C 1G H 31 O) 3 O 3 , which is a solid fat, 

 and olein, C 3 H 5 (C 18 H 33 0) 3 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 liuoleic acid) and palmitin. 



5. Essential or Volatile Oils : these substances occur in various parts of 



