2 INTRODUCTION [ch. 



The lecithins, which are compounds of fats with phosphoric acid, are 

 probably present in all living cells and have an important metabolic 

 significance. 



The above substances belong to the aliphatic series of organic com- 

 pounds, that is to the series in which the carbon atoms are united in 

 chains. 



(4) Aromatic compounds. These are characterized by having the 

 carbon atoms united in a ring as in benzene. They may contain more 

 than one carbon ring, and, moreover, aliphatic groupings may be attached 

 to the carbon ring as side-chains. The number of aromatic substances is 

 very great, and every plant contains representatives of the class. Some 

 members are widely distributed ; others, as far as we know, are restricted 

 in their distribution, and may be peculiar to an order, a genus or even 

 a species. This class contains: (a) Phenols, i.e. hydroxy-derivatives of 

 T^enzene, such as phloroglucinol. (h) Aromatic alcohols, aldehydes and acids 

 derived from benzene ; various hydroxy-benzoic acids, such as gallic and 

 protocatechuic acids, are important, since, by condensation, they give 

 rise to tannins. Just as in the case of the carbohydrates, where simpler 

 compounds may become more complex by condensation, the soluble 

 crystalline acids condense to form the complex colloidal tannins. Of 

 other aromatic acids, the amino derivatives, such as phenylalanine and 

 tyrosine, form constituents of proteins, (c) Complex hydrocarbons, the 

 terpenes, accompanied by derivative alcohols, aldehydes, ketones and 

 esters. These form constituents of the "essential oils" obtained from 

 plants by steam distillation, and are responsible for most of the plant 

 scents, (d) Other members which contain more than one ring are the 

 water-soluble yellow, red, purple and blue pigments of plants, the yellow 

 being hydroxy -flavones and flavonols, the remainder, anthocyan pigments. 



(5) Proteins. This large class contains substances which are in many 

 cases built up of groupings from both the aliphatic and aromatic series. 

 It includes not only the proteins but also their simpler derivatives, the 

 albumoses, peptones and polypeptides. In this case, as before, the simplest 

 derivatives, known as the amino-acids, are synthesized by condensation 

 to form the polypeptides, peptones, albumoses and proteins, in a series 

 of increasing complexity. The amino-acids are compounds, either of the 

 aliphatic, aromatic or heterocyclic (see 6) series, in which one or more 

 hydrogen atoms are replaced by the radicle NHg. They are soluble and 

 crystalline, but after condensing together, the final product, the protein, 

 only exists in either the solid or the colloidal state. Proteins, in the 

 latter condition, constitute the bulk of the complex material, protoplasm ; 



