366 VEGETABLE PHYSIOLOGY 



As we have seen, the environment of the plant is partly 

 the soil and partly the atmosphere, and the temperature of 

 both may or may not vary simultaneously. We have 

 seen that for each metabolic process there is a temperature 

 at which it progresses to the greatest advantage. At lower 

 and at higher points the protoplasm is less active, and in 

 each case there is a point below which activity ceases, and 

 one above which also it does not go on. The same thing 

 we have seen is true of the processes of growth. We may 

 say that for each plant there is a particular temperature at 

 which it carries out the aggregate of its functions most 

 advantageously, and it is when exposed to this temperature 

 it is in a condition of the most complete thermotonus. 

 This point is not the same for every plant, indeed consider- 

 able differences exist in this respect. We may say more- 

 over that it is perhaps not so much a point as a range of 

 temperature, for small divergences from the actual optimum 

 point have but little effect upon the tone. Within this 

 range the constant round of activity, chemical and physical, 

 which is the expression of life, goes on most advantageously, 

 below it it is injuriously affected, and at a minimum point 

 it is suspended. At another point, higher in the scale, 

 spoken of as the maximum temperature, the death of the 

 protoplasm usually ensues. 



We cannot explain the influence of temperature upon 

 the protoplasm any more satisfactorily than we can that of 

 light. All we know is that the two co-operate together to 

 keep the plant in the condition to which we have given the 

 name of health. 



The tone of the plant depends very greatly upon a 

 proper adjustment of the relations between the protoplasts 

 and water. For the maintenance of health it is essential 

 that the normal turgidity of the cells shall not be disturbed. 

 A definite amount of hydrostatic pressure inside such cells is 

 necessary as we have seen for the due or efficient discharge 

 of the processes of life. We may regard the maintenance 

 of this relationship as one of the chief features of tone, 



