FHYSICOCHEMICAL ORGANIZATION OF THE PLANT 9 



empty skull, they force it to separate along the sutures of the 



skull bones. 



With increasing water content, the water-attracting force of 

 the jelly gradually diminishes and finally becomes zero. This 

 will be at the point of complete saturation with water. Some 

 colloids retain the character of hard though gelatinized bodies 

 even in a state of complete saturation and remain sharply distinct 

 from the surrounding water. These are colloids with a limited 

 swelling capacity, such as cellulose and starch. Protoplasm 

 also belongs to the number of colloids that, though hquid, are 

 limited in their swelhng capacity. When in a state of complete 

 saturation or even somewhat earher, some jelUes begin to show 

 symptoms of dissociation of their micelles and finally dissolve 

 completely in the solution. Such are, for instance, many protein 

 substances, and gums (gum arable). They are designated 

 as colloids with unlimited swelhng capacity. There is no sharply 

 defined boundary between colloids with limited and unhmited 

 swelling capacity. Thus, for instance, at a low temperature 

 gelatin swells to a certain extent, while at increased temperatures 

 it swells without hmit. When heated, starch forms a jelly that 

 changes into a colloid with unlimited swelling capacity. 



The phenomena of swelling and dehydration play very impor- 

 tant parts in the life of the cell. Most of the vital processes take 

 place only in a protoplasm sufficiently swollen and practically 

 saturated with water. 



Both the cell wall and the protoplasm are comphcated colloid 

 systems, built up of ultramicroscopic elements invisible even 

 under the most powerful microscope. The study of this struc- 

 ture requires therefore the apphcation of ultramicroscopic and 

 X-ray methods of investigation as well as the study of surface 

 tension, viscosity, elasticity, and its other physicochemical 

 properties. As yet all the details of the complex structure of 

 these colloid systems are not fully known. Recent investiga- 

 tions, however, have revealed the more important basic features 

 of their structure. 



The cell wall, at least of the younger cells, consists mainly of 

 cellulose, the chemical composition of which will be discussed 

 somewhat later in Chap. II. At present it will only be indicated 

 that cellulose is a polysaccharide of high molecular weight. 

 It is built up of cychc molecules of anhydroglucose (CeHioOo) 



