124 TEXTBOOK OF PLANT PHYSIOLOGY 



metal, acids, alkalies, as well as by an excessive withdrawal of 

 water or even by mechanical pressure. 



In all cases, coagulation leads to death of the protoplasm. 

 This shows that a comparatively high degree of dispersion of the 

 colloids making up protoplasm is closely connected with its vital 

 properties. 



The phenomenon of coagulation is peculiar not only to proto- 

 plasm and albuminous substances in general, but also to other col- 

 loids of emulsoid and of suspensoid character. With suspensoids 

 is obtained almost always an irreversible coagulation. In many 

 emulsoids, on the contrary, the precipitated or dried colloid may 

 return to the condition of a gel. This kind of coagulation is called 

 reversible. The same colloids may show in some cases a reversible, 

 in other cases, an irreversible coagulation. Thus, the white of an 

 egg coagulates to an irreversible state when heated or when treated 

 with poisons. When dried or subjected to the influence of strong 

 solutions of neutral salts, it enters the condition of a gel, which may 

 be dissolved again. 



In many instances, protoplasm may be dried to a state of com- 

 plete air desiccation when usually 10 to 12 per cent of so-called 

 hygroscopic moisture remains without losing its vitality, i.e., the 

 faculty of returning to the sol condition. Thus, seeds which in 

 ripening lose all free water and become air dry, simultaneously 

 suspend all vital processes. When moistened, however, they 

 once more return to activity. The same is true of many crypto- 

 gams, such as mosses, lichens, and some ferns. Not only their 

 organs of reproduction, but the whole thallus of these plants may 

 be reduced to an air-dry condition. They return to activity when 

 moistened. In such a dried protoplasm, however, some processes 

 must be going on which alter the dispersion of its particles. When 

 stored for a very long period, dry seeds lose their power of germi- 

 nation. Under similar conditions, dried cryptogams lose their 

 capacity of returning to life. Similar coagulation is observed 

 when photographic plates are kept for a long time. Their emul- 

 sion gradually becomes more and more coarse grained and the 

 plates lose their sensitivity to light. 



40. Influence of Low Temperatures on Plants. Freezing and 

 Frost Killing. — Changes in the condition of the colloidal substances 

 of the protoplasm supply a clue to the knowledge of the complex 

 phenomena taking place in the plant at temperatures below freez- 



