INTRODUCTION [CH. 



< 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 the various phenols, i.e. hydroxy- 

 derivatives of benzene, such as phloroglucin; and the corresponding acids, 

 i.e. hydroxy-derivatives of benzoic acid, such as gallic and protocatechuic 

 acids. Just as in the case of the carbohydrates, where simpler compounds 

 may become more complex by condensation, so the aromatic acids, 

 aldehydes, and their derivatives may be condensed to form more complex 

 substances, such as tannins. Other members containing more than one 

 benzene ring are the water-soluble yellow, red, purple and blue pigments 

 of plants, the yellow being hydroxy-flavones and flavonols, the remainder 

 V anthocyanins. The simplest aromatics occur in true solution in the 

 cell-sap throughout the plant, and the more complex ones exist in the 

 colloidal state. 



Another section of aromatics is represented by the "essential oils." 

 They are heterogeneous in chemical composition, though they are 

 chiefly represented by the hydrocarbons of the terpene series. They also 

 include various alcohols, aldehydes and ketones. They have no relation- 

 ship with the true fats, but are responsible for many of the well-known 

 scents of plants ; as examples may be mentioned limonene in lemons, 

 pinene in Conifers, borneol in Thyme and Rosemary, menthol in 

 Peppermint and camphor in the Camphor tree (Laurus Camphora). 



(4) 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 or aromatic series, in which one or more hydrogen atoms 

 are replaced by the radicle NH 2 . 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 ; in the solid 

 state, in the form of grains and granules, they occur as reserve material 

 in the cell. 



The above-mentioned classes will be dealt with in greater detail in 

 the later chapters. There are, of course, a large number of other 



