186 PART III. PHYSIOLOGY. [ 45 



air contain from 12 to 15 per cent, of water, herbaceous plants 60 

 to 80 per cent., and many water-plants and Fungi as much as 95 

 per cent, of their whole weight. 



The residue, which gives off no more water at a heat of 100 C., 

 the dry solid, consists of a great variety of chemical compounds ; 

 these are partly organic, that is to say, combinations of carbon 

 w r ith other elements, and partly inorganic. The organic sub- 

 stances which occur in the living plant (with the exception of 

 salts of oxalic acid) all contain hydrogen. Some of them, such as 

 many oils, consist of these two elements only (carbon and hydro- 

 gen), but by iar the greater number, including cellulose, starch, 

 and sugar, as well as the vegetable acids and certain oils, contain 

 oxygen also. The proteid substances consist of carbon, hydrogen, 

 oxygen, nitrogen, sulphur, and sometimes phosphorus ; in other 

 bodies which contain nitrogen, as asparagin and many alkaloids, 

 there is no sulphur or phosphorus ; from certain other alkaloids, 

 for instance nicotin, oxygen is also absent. 



The commoner organic substances of which the plant-body 

 consists may, in the first instance, be divided into those which 

 do and those which do not contain nitrogen in their molecule. 



The most important nitrogenous substances may be classified as follows : 



1. Proleids : these are substances with a large molecule of complex con- 

 stitution, to which no chemical formula has yet been assigned ; they may 

 be soluble or insoluble in water, and when soluble are mostly indiffusible ; 

 they are generally of a viscid nature (like white of egg) and are rai-eiy 

 crystal] isable. 



2. Amides (or Amido-acids): these substances are soluble in water, not 

 coagulated on boiling, diffusible, and crystallisable. Those commonly 

 occurring in plants are Asparagin (C 4 H 8 N n O 3 ), Leucin (C 12 H 26 N 2 O 4 ), Tyrosin 

 (C 9 H n N0 3 ). 



3. Alkaloids: these substances are, chemically, organic bases, occurring 

 in plants in combination with organic acids ; they are insoluble or but 

 slightly soluble in water, soluble in alcohol ; most of them are solid at 

 ordinary temperatures, and are crystalline, whilst others are liquid (Coniin, 

 Nicotin) ; they are generally poisonous. 



The more familiar alkaloids are Coniin (C 8 H 15 N) from Conium ; Nicotin 

 (C IO H 14 N 2 ) from Tobacco ; Morphin (C 17 H 19 NO 3 ), and other opium-alkaloids 

 from the Poppy ; Strychnin (C 2 iH 22 N 2 O 2 ) from Strychnos Nux vomica; Quinin 

 (C2oH 24 N 2 O 2 ) from the Cinchona; Thein (C 8 Hi N 4 O 2 ) from Tea; Theobromin 

 (C 7 H 8 N 4 O 2 ) from Theobroma Cacao. 



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

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



The principal non-nitrogenous substances are : 



1. Carbohydrates : substances consisting of C, H, and O, the H and O be- 



