TONE 365 
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 goon. 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 
that 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 ag one of the chief features of tone, 
