TONE 359 
relationship between the plant and temperature may be 
called Thermotonus. This condition also is necessary for 
the manifestation of sensitiveness. If it is materially 
interfered with, the vital functions and the processes of 
growth and nutrition suffer seriously. 
There must also be a satisfactory adjustment of the 
relations between a plant and moisture, though this is less 
restricted than the two already mentioned. 
As the maintenance of health involves the continual 
adjustment of the plant to the changes in its environment, 
we must examine a little more closely the nature of the 
influence which the latter, and particularly the two factors 
of light and temperature, exert upon the organism. This 
influence is spoken of as a tonic or paratonic influence and 
leads to the establishment of a satisfactory condition of 
tone. ; 
In order to study the tonic influence of light upon a 
plant we may first consider the features which characterise 
the growth of a plant in darkness. We find that such a 
plant is much modified both in form and structure. If we 
experiment with an ordinary dicotyledonous plant which 
has numerous leaves of moderate or small size upon an 
elongated stem, we find that these features become much 
exaggerated. The stem becomes very much elongated and 
remains slender ; it is more succulent than a normal stem, 
and bears extremely small leaves which grow out’ from it 
at a more acute angle than those which rise upon a 
normally illuminated stem. Certain Monocotyledons which 
have normally small stems and large broad leaves are 
differently affected. The great change in this case is in 
the leaves, which become much elongated and relatively 
narrower than normal ones. Certain phylloclades, such 
as those of some of the Cacti, become elongated and slender, 
instead of remaining broad. 
The structure of the various parts also is modified; the 
woody and sclerenchymatous elements are much reduced, 
and the parenchyma of the cortex is increased in bulk. It 
