CHAPTER II. — SPECIAL PHYSIOLOGY OF THE NUTRITIVE FDNCTIONS. 707 



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

 gen), but by far the greater number, including cellulose, starcb, 

 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. Proteidii : these are substances with a large molecule of complex constitu- 

 tion, 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 gener- 

 ally of a viscid nature (like white of egg) and are rarely crystallisable. Of these 

 there are several varieties : — 



a. Insoluble Proteids : do not dissolve even in dilute acids or alkalies : e.g. 



coagulated proteid. 



b. Albuminates : insoluble in water or solutions of neutral salts, but soluble 



in dilute acids or alkalies, sometimes soluble in dilute alcohol : e.g. 

 gluten of wheat. 



c. Globulins : msoluble in water, but soluble in solutions of neutral salts 



(such as NaCl) and coagulated on boiling, sometimes crystallisable : 

 these enter largely into the composition of aleuron (see p. 112) ; the 

 crystalloids in the potato and in some aleuron-grains {e.g. Ricinus, 

 BerthoUetia) consist of a kind of globulin (vitellin). 



d. Albumins : soluble in water, coagulated on boiling : rare. 



e. Albumoses : soluble in water, not coagulated on boiling, but precipitated by 



a small quantity of nitric or acetic acid, and soluble in excess : a common 

 constituent of aleuron. 

 /. Peptones : soluble in water, not precipitated by boiling or by acids ; present 

 in germinating seeds. 



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 (C4H8N2O3), Leucin (<^i2H23N204), Tyrosin (C9H11NO3). 



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 tem- 

 peratures, and are crystalline, whilst others are liquid (Coniin, Nicotin) ; they 

 are generally poisonous. 



The more familiar alkaloids are Coniin (CgHjjN) from Conium ; Nicotin 

 (CioHi^Nj) from Tobacco ; Morphin (CiyHioNOg), and other opium-alkaloids 

 from the Poppy; Strychnin (CoiHj^NoOj) from Strychnos Nux vomica ; Quinin 

 (020^21X202) from the Cinchona; Thein (C8H10N4O2) from Tea; Theobromin 

 (C7 Hg N4 O2) from Theobroma Cacao. 



