i] INTRODUCTION 3 



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

 material in the cell. 



(6) Plant bases. This class contains (a) the amines or substitution 

 products of ammonia. Sometimes the hydrogen of ammonia is substi- 

 tuted by a group of some complexity which leads to the production of a 

 ■compound of the heterocyclic type, i.e. with a ring containing both carbon 

 and nitrogen atoms. The pyrrole ring is an example which occurs in the 

 amino-acid, proline, in certain alkaloids (see below), and in the pigment 

 chlorophyll, (b) Purines. In connection with these substances we need 

 to consider two more heterocyclic rings, i.e. the pyrimidine and the 

 iminazole. The former may be regarded as the condensation product of 

 urea, which is possibly present in small quantities in plants, and an un- 

 saturated acid, e.g. acrylic acid. The pyrimidine ring is present in some 

 purines, the iminazole in the amino-acid, histidine. The remaining 

 purines contain a condensed pyrimidine and iminazole ring. Certain of 

 the purines become condensed together, in combination with phosphoric 

 acid and a pentose sugar, to form the nucleic acids. The latter, in com- 

 l)ination with proteins, as nucleoproteins, form a constituent, as their 

 name implies, of the nucleus, (c) The alkaloids are substances of con- 

 siderable complexity, containing various heterocyclic rings. Unlike the 

 simpler bases, they are restricted to a certain extent in their distribution. 



It is not possible to include all classes of plant substances in the 

 above list and many others, such as the sulphur compounds, sterols, 

 phytin, etc., are referred to in the later chapters. It should be borne in 

 mind that the importance of a compound in plant metabolism is not 

 estimated by the amount of it occuring in the plant. Frequently, most 

 important substances occur in such small quantities that they are diffi- 

 cult to detect. 



In order to appreciate the subject of plant chemistry, the plant, 

 which is familiar as a botanical entity, must be interpreted in chemical 

 terms. The principal classes of the more essential and widely distributed 

 compounds found in plants have already been indicated on the broadest 

 basis, so that they may now be referred to without additional comment. 



From the botanical point of view, the plant may be regarded as a 

 structure composed of many living protoplasmic units enclosed in cell- 

 walls and combined together to form tissues. There are also certain 

 tissues, known as dead tissues, which assist in giving rigidity to the plant. 

 All these structural elements may, in time, be translated into terms of 

 chemical compounds. 



1—2 



