TONE 367 
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 
hgs~aumerous leaves of moderate or small size upon an 
ae stem, we find that these features become much 
exaggorated, The stem becomes very much elongated and 
remains eae it is more succulent than a normal stem, 
and’ ‘bears extremely small leaves which grow out from it 
at a more acute ee than those which rise upon a normally 
illuminated ste Certain Monocotyledons which have 
normally small stl ms and large broad leaves are differently 
affected. at 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 
becomes more succulent, and the reaction of its sap is much 
more acid. The chloroplasts do not become green, the 
