708 PART IV. — THE PHYSIOLOGY OF PLANTS. 



Some colouring -matters are also nitrogenous {e.g. chlorophyll, and indigo 

 CgHjNO), as also some glucosides (see below). 

 The principal non-nitrogenouH substances are : — 



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

 present in the same proportions as in water (HgO) ; of these there are the 

 following classes : 



a. Amylases : general formula n (CqH^qO^) ; 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 ainylin, a product of the action of diastase on starch, 

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

 positse and allied orders (Campanulace», Lobeliaceee) 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 : Ci2H220ii : 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 : CeHjoOe : soluble in water and crystallisable : they occur in 



fruits (grape-sugar). 



The sucroses and glucoses are commonly known as sugars. 



A substance termed Mannite {G^-^fi^ occurs in the cell-sap of Fraxinus 

 Onius 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 commonlj', 

 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 (H2C2O4), malic acid (H2C4H4O5), 

 tartaric acid (H2C4H4O6), citric acid (H3CeH507). 



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 amygdaiin, C20H27NO11 (seeds, etc., of many Kosaceae) ; coniferin, 

 CieHaaOg (coniferous wood) ; my rosin, or myronate of potash, KCioHiaNSgOio 

 (seeds of Mustard) ; salicin, C12H18O7 (in bark of Willows and Poplars) ; gallo- 

 tannin, O-^^.^^O.^^ (in Oak-bark). 



Though some of these substances {e.g. amygdaiin 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 plants, 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 plycerides palmitin, C3H5(Ci,5H3iO)303, which is a solid fat, 

 and olein, C3H5(Ci8H330)303, 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 



