I] INTRODUCTION 3 



substances in the plant, some of which may be inchided with the above, 

 while others form additional small classes, as for instance, the 

 alkaloids : others, again, bear no close relationship to any class, such as 

 the all-important pigment, chlorophyll. 



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 distri- 

 buted 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 can be translated into terms of 

 chemical compounds. 



One of the chemical processes most frequently met with in the plant 

 is that of synthesis by condensation, with elimination of water, of large 

 complex molecules from smaller and simpler molecules. The formation 

 of cellulose, for instance, is a case in point. Cellulose has the composition 

 (CbHioO,,),! and, on hydrolysis with dilute acids, it yields glucose as a final 

 product. Hence it is concluded that the complex molecule of cellulose is 

 built up from the simpler carbohydrate by condensation. The synthesis 

 of proteins from amino-acids affords another example. These acids 

 contain either an aliphatic or aromatic nucleus (let it be R), and one or 

 more carboxyl and amino groups. Condensation takes place in the plant, 

 with elimination of water, according to the following scheme : 



Ri Rii Riii RX 



I i I _ I 



NH, CH~COOH HNH CH— COOH H NH CH -CO OH HNHCH— COOH 



The products of such condensation, the proteins, vary among them- 

 selves according to the number and kind of amino-acids which take part 

 in the synthesis. 



Two important r'esults arise from this process. First, the substances 

 formed by condensation have molecules of a very large size ; secondly, 

 whereas the simple compounds, sugars and amino-acids, are soluble, 

 crystalline and diffusible, the condensation products are either insoluble, 

 e.g. cellulose, or exist in the colloidal state, as is the case of many proteins 

 and other plant constituents. As these very large molecules do not dialyze, 



1—2 



