360 VEGETABLE PHYSIOLOGY 



A corresponding condition marking an appropriate 

 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 par atonic 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 is 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 arise 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 



