4 INTKODUCTION [cr. 



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 

 (CeHioOs)^ 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 con- 

 tain 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 Ra; 



i , I I I 



NHoCH— COiOH HInH-CH— COiOH HiNH • CH— COiOH HiNHCH— 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 results 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> 

 they remain where they are synthesized, and build up the solid structure 

 of the plant, as for instance, the cell-walls. 



Matter in the colloidal state is of very great importance in the plant 

 and is probably responsible for many of the properties of ** living'^ 

 material. Thus it will not be out of place, though it will be referred to 

 again in a later chapter, to make at this point a few remarks on the 

 colloidal state. It has been known for some time that certain metals, 

 e.g. gold and silver, and also certain metallic hydroxides and sulphides, 

 e.g. ferric hydroxide and arsenious sulphide, though insoluble in water 

 under ordinary conditions, can, by special methods, be obtained as solu- 

 tions which are clear to the unaided vision. Such solutions are termed 

 colloidal. Investigation has shown that the matter is not present in true 

 solution, but in a very finely divided state, i.e. as particles many times 

 larger than simple molecules, but smaller than the particles obtainable 

 by mechanical means of division. Such solutions are known as artificial 

 colloidal solutions, but there are a number of organic substances, with 

 very large molecules, such as proteins, starch, gums, agar, etc., which at 



