THE VITAL PROPERTIES OF THE CELL 97 



lowered, as the case may be, the vital appearances gradually become 

 normal. It is true that if the condition of rigor produced by cold 

 is maintained for a considerable time, death may ensue, although 

 cold is better withstood, and for a longer time, than heat. When 

 the protoplasm dies it becomes coagulated and turbid, whilst com- 

 mencing to swell up and to decompose. At the temperatures lying 

 between these extremes, the vital processes are performed in a 

 manner which varies in intensity with the degree of temperature. 

 This is especially true of the movements which take place at dif- 

 ferent speeds, increasing in rate up to a certain point, as the tem- 

 perature rises, until they reach a certain fixed maximum speed. 

 This occurs at the so-called optimum temperature, which is always 

 several degrees below that at which heat rigor is produced. As 

 the temperature passes this limit, the protoplasmic movements are 

 seen to slacken, until at last rigor sets in. 



White blood corpuscles have been much used in studying the 

 effects produced by heat ; for this purpose Max Schultze's warm 

 stage, or Sachs' warm cells, are most suitable. In a fresh drop of 

 blood the corpuscles are seen to be motionless and globular in 

 form. If the drop is warmed the necessary precautions being of 

 course observed the corpuscles gradually commence to extend 

 pseudopodia, and to move about. As the temperature approaches 

 the optimum for the time being, these changes of shape become 

 more rapid. In Myxomycetes, Rhizopoda, and plant cells, the 

 effect produced by an access of heat is exhibited by an increase of 

 rapidity of the streaming movements of the granules. Thus, 

 according to the measurements of Max Schultze (I. 29), the 

 granules in the hair-cells of Urtica and Tradescantia travel at 

 ordinary temperatures at a rate of , 004- , 005 mm. per second, 

 whilst if the temperature is raised to 85 C, their speed is in- 

 creased to '009 mm. per second. In Vallisneria the rate of 

 circulation may be increased to *015 mm., and in a species of 

 Char a even to '04 mm. per second. The difference between the 

 slow and accelerated movements may be so great that whilst with 

 the former the length of a foot is traversed in fifty hours, with 

 the latter the same distance may be covered in half an hour. 



Nageli (III. 16) has expressed the acceleration produced by an 

 accession of heat in the grranular streaming movements in the cells 

 of Nitella by the following figures : in order to traverse a distance 

 of T mm. the granules require 60 seconds at 1 C. ; 24 seconds at 

 5 C. ; 8 seconds at 10 C; 5 seconds at 15 C. ; 3*6 seconds at 



H 



