866 VEGETABLE PHYSIOLOGY 



This property is not always to be observed or demon- 

 strated with equal ease. Indeed, the protoplasm must be in 

 a healthy condition to manifest it satisfactorily. It is easily 

 injured if changes in the environment are too sudden or too 

 severe. Consequently the adaptation of groups of plants to 

 special environments has been a slow and difficult process, 

 any single individual undergoing little change, but altera- 

 tions of considerable extent have been effected by the con- 

 tinuous influencing of many generations. 



The maintenance of the health of the individual is no 

 doubt the great object of this sensitiveness ; and conversely 

 it is only the healthy plant that manifests it in the greatest 

 fulness. Health may be spoken of as the condition in 

 which the reaction between an organism and its surround- 

 ings is a perfect one. In the case of the ordinary terrestrial 

 plant these surroundings present especially three features 

 which are subject to considerable variation. These are 

 light, temperature, and moisture. A plant must exhibit 

 a proper relationship to each of these conditions, at any 

 rate, to be healthy. The condition in which the relationship 

 to each of these factors is satisfactory is generally spoken 

 of as one of tone, and the influence which each exerts when it 

 affects the plant uniformly is spoken of as a tonic influence. 

 When a dicotyledonous plant which has been growing under 

 ordinary atmospheric conditions, exposed to diffused day- 

 light, is removed into darkness and kept there for some 

 time, it becomes incapable of being impressed by its sur- 

 roundings. Nor is its irritability alone affected by the 

 absence of light, for many of its parts, particularly its leaves, 

 cease to grow under such conditions. The condition which 

 is induced by light, and upon which the manifestations of 

 irritability depend, is known as Phototonus. It indicates 

 a certain effort on the part of the protoplasm to adjust 

 itself to the intensity of the illumination. 



A corresponding condition, marking an appropriate 

 relationship between the plant and temperature, may be 

 called Thermotonus. This condition" also is necessary for 



