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 

 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 

 becomes more succulent, and the reaction of its sap is much 

 more acid. The chloroplasts do not become green, the 



