494 PROTOPLASM 



concentrated and may occur elsewhere. Gelatinization (reversi- 

 ble gelation) of the protoplasm takes place at a more distant 

 point where the concentration of the salt is not too great. In 

 Fig. 51, coagulated protoplasm is to be seen at the point of 

 injection and in the tip of the root hair ( a bridge of protoplasm 

 is formed in the center between two vacuoles). Where irreversi- 

 ble coagulation is slight, the cell may fully recover in twenty to 

 thirty minutes through solation of the reversible protoplasmic 

 jelly. As the latter disintegrates, bits of it are carried in the 

 protoplasmic stream. 



The Coagulation of Blood. — Blood clotting is a case of pure 

 coagulation and probably the event that gave rise to the term. 

 It is nature's way of preventing excessive loss from a wound. 

 Mere contact with air is sufficient to accomplish it, and the process 

 is irreversible in the strict sense. 



While blood clotting must be regarded as an example of coagula- 

 tion, its mechanism appears to be wholly distinct from any of 

 the foregoing processes, such as suppression of the surface 

 potential by ions, mutual decharging of colloidal particles, heat, 

 dehydration, and denaturization (decreased solubility due to 

 adsorption). (Fibrinogen is very readily coagulated by heat, at 

 as low a temperature as 56°C., but the natural coagulation of 

 fibrinogen in blood is not by this method.) Blood consists of 

 red corpuscles, white corpuscles, and platelets in suspension in a 

 plasma containing fibrinogen and serum proteins. The white 

 corpuscles are alive. Whether or not the red corpuscles and 

 platelets are to be so regarded is an open question usually 

 answered in the negative. Through the work of Morawitz, 

 Bordet, Howell, Eagle, and others, it has been definitely estab- 

 lished that at least four substances are actively concerned in 

 blood coagulation — fibrinogen, calcium, blood platelets, and an 

 unknown plasma substance, termed thromhogen, or prothrombin. 

 The first — fibrinogen — is the substance coagulated, being con- 

 verted into fibrin. The other three interact to form thrombin, 

 the actual coagulating substance; thus, the soluble protein 

 fibrinogen is converted by thrombin into the insoluble protein 

 fibrin, a fibrous coagulum. 



The function of calcium in this process is unknown. It may 

 be an intrinsic active part of thrombin. The fact that thrombin 

 to which oxalate or citrate has been added is still active does not 



