106 TEXTBOOK OF PLANT PHYSIOLOGY 



of that of sols. The original dispersed substance builds a contin- 

 uous mass in which the remains of the former dispersion medium 

 appear as the dispersed phase. 



Sometimes the transformation of a sol into a gel is effected 

 without the removal of the greater part of the dispersion medium. 

 Thus, for instance, the congealing of gelatine takes place with the 

 cooling down of the sol to ordinary temperature. Such gels sat- 

 urated with water are called "jellies." There is reason to suppose 

 that the colloidal substance of such jellies forms something of the 

 nature of a complex network, in whose meshes the water is retained 

 in an immobile condition. 



When dried, jellies decrease markedly in volume, losing their 

 water content. But only the first portions of water are given up 

 as readily as in colloidal solutions. Further loss of water meets 

 with progressively increasing resistance. The last portions of 

 water may be removed only by heating to 100° C. On the other 

 hand, when a dried jelly is immersed in water, it imbibes the latter, 

 and increases its volume rapidly. This phenomenon is termed 

 swelling of the colloid. The first portions of water are imbibed 

 by the dried colloid with a force as high as 1,000 atmospheres. 

 The weight that the swelling colloid is able to lift may serve as a 

 measure of this force. Especially convenient for such experiments 

 are dry seeds, which in swelling develop a pressure great enough 

 to burst a bottle. When such seeds are placed in an empty skull, 

 their pressure causes the bones to separate at the sutures. 



As the water content of the gel increases, its water-absorbing 

 power gradually decreases, finally becoming zero. In this condi- 

 tion the colloid ceases to attract water. This is a state of perfect 

 saturation. Some colloids preserve in this condition the charac- 

 ter of solid though jelly-like, bodies, and therefore are sharply 

 distinguished from the surrounding water. Such colloids of lim- 

 ited swelling capacity are, for instance, cellulose and starch. To 

 this group also belongs protoplasm. Other colloids begin to lose 

 the attraction between their particles when perfectly saturated 

 with water, or even before this stage is reached, and are gradually 

 converted into a solution. Such colloids of unlimited swelling 

 are, for example, the protein substances and the gums as gum 

 arabic. There is no sharp line of demarcation between colloids 

 of limited and colloids of unlimited swelling. The swelling of gela- 

 tine, for instance, is limited at low temperatures and unlimited at 



