22 Practical Plant Biology. 



concentrate on a surface in contact with a solution, this sur- 

 face condensation, or adsorption as it is called, becomes very 

 important in the case of colloids. 



In a liquid dispersion of gelatine in water, the water forms 

 the continuous phase, i.e. it is continuous, while the gelatine, 

 which is in the form of excessively minute, isolated particles, 

 is the discontinuous phase. As more and more water is ab- 

 stracted from such a dispersion, a critical concentration is reached 

 when the condition of things is reversed and the gelatine becomes 

 the continuous phase, holding the water enclosed in it as a 

 dust of minute particles. While the water is the continuous 

 phase the mixture acts like a liquid and is called a sol, but 

 when the gelatine is continuous the mixture resembles a solid 

 and, it is called a gel. The transition from sol to gel at any 

 given concentration of the colloidal substance depends on the 

 temperature. Thus a dispersion of gelatine which is a sol at 

 40 C. may be a stiff gel at 20 C. An item of great interest 

 attaches to the melting-point of a gel. When the temperature 

 at which a gel breaks down and becomes a sol is noted and 

 compared with that at which the same dispersion becomes a 

 gel on cooling, the two do not correspond. The melting-point 

 is different according as it is approached from a higher or a 

 lower temperature. In fact, the past history of the colloidal 

 mixture controls its behaviour. 



In the cells of plants cellulose, starch and sometimes protoplasm 

 are in the gel state and so may be compared to a honeycomb 

 enclosing droplets of water. The proteins and other colloids of 

 the vacuole and often protoplasm are examples of the sol state, 

 and are distributed through the water as an inconceivably fine 

 dust. The colloid in either condition presents an enormous 

 surface. The gel state may pass automatically by simple dilution 

 into the sol condition, and vice versa by concentration pass from 

 the sol to the gel condition. Vegetable gums furnish examples 

 of these changes. Sometimes such reversal is dependent on heat, 

 as in the case of gelatine. Another change, which is exhibited by 

 some bodies in the colloidal state, is coagulation or precipitation. 

 It is shown in a characteristic manner by proteins and protoplasm. 

 In it the transparent sol or gel becomes opaque and the colloidal 

 substance aggregates like a precipitate. Again, if a semi-perme- 

 able membrane formed of a protoplasmic gel is coagulated it 

 becomes permeable. Coagulation may be brought about by heat. 

 A temperature of about 70 C. is sufficient to coagulate many 

 proteins and protoplasm. Here may be recalled the observation 



