4 METABOLISM 



We may distinguish : 



1. Organic acids. Many of these by their very names show that they were 

 primarily discovered in plants, e.g. oxalic, malic, tartaric, and citric acids, 

 although they are by no means confined to the species whence they derive 

 their names. The lower members of the fatty acid series are also very frequently 

 met with in plants, e. g. formic, acetic, propionic, and butyric acids. 



2. The glycerides of the higher fatty acids are known as fats, especially 

 the glycerides of palmitic, stearic, and oleic acids. Suberin also is a glycerine 

 compound of a fatty acid (suberic acid) and may for that reason be included here. 

 Again the various vegetable waxes belong to the same category, for most of them 

 are true fats or glycerine esters, some, however, are esters of univalent alcohols 

 with fatty acids. Finally we may add lecithin and cholesterin, which have many 

 characters in common with fats, but which have a more complex composition. 



3. Among the carbohydrates we may note first of all the monosaccharides, 

 which contain six carbon atoms (hexoses), such as glucose (dextrose), mannose, 

 galactose, and levulose, or only five (pentoses), such as xylose and arabinose. 

 The disaccharides have a larger molecular composition, and these bodies, by 

 taking up water, hydrolyse easily into two hexose molecules ; e. g. cane-sugar 

 decomposes into dextrose and levulose, milk-sugar into dextrose and galactose, 

 maltose into two molecules of dextrose. The largest molecule occurs in the 

 polysaccharides (starch, cellulose, &c.), which can be decomposed into several 

 hexose and even pentose molecules. 



4. Amido-compounds, i. e. amido-acids and acid amides. The amido-acids 

 are derived from fatty acids by the substitution of NH 2 for H ; e. g. aspartic 

 acid = amido-succinic acid ; leucin = amido-caproic acid ; alanin = amido- 

 propionic acid, occurring especially in conjunction with phenol to form tyrosin. 

 The acid-amides arise by substitution of NH 2 for OH in carboxyl ; e. g. asparagin 

 = amido-succinic-acid-amide ; glutamin = glutaminic-acid-amide. 



5. Etherial oils are familiarly recognized as the oily, volatile substances 

 which are the source of many vegetable perfumes. From the chemical point 

 of view we may distinguish (a) the terpins, simple hydrocarbons, e. g. of oil of 

 turpentine and the oils which occur in the Myrtaceae and Umbelliferae ; to 

 which group belong also caoutchouc and its relative guttapercha, the latter 

 differing from the former, however, in having oxygen in its composition ; 

 (b) oxygen-containing bodies, such as camphor and many of the oils of the 

 Labiatae ; (c) etherial oils containing sulphur, such as those of certain species 

 of Allium and of the Cruciferae. 



6. The resins are related to the etherial oils, in which, as a matter of fact, 

 they are not infrequently dissolved ; these bodies are chemically difficult to 

 determine (TscHiRCH, 1900). 



7. The alkaloids are nitrogenous plant bases, familiar to us owing to the 

 fact that to them may be attributed the poisonous properties possessed by very 

 many plants. Their physiological significance is as yet but little known. 



8. The glucosides are readily distinguished by the ease with which they 

 can be decomposed into hexoses and various aromatic substances. Thus the 

 nitrogenous substance amygdalin, found in bitter almonds, decomposes into 

 glucose, oil of bitter almonds and hydrocyanic acid, the non-nitrogenous salicin 

 gives saligenin and glucose. Many tanning materials also are related to the 

 glucosides and yield by decomposition, in addition to gallic acid, a sugar or the 

 ' aromatic sugar ' phloroglucin. These substances concern us, as physiologists, 

 but slightly. 



9. The pigments in the plant are both chemically and physiologically 

 extremely varied. We need only note here chlorophyll as the most important 

 of them. 



10. The proteids are at once the most important and also the most complex 



