PEOPEETIES OF VEGETABLE PEOTOPLASM 359 



organism. It is usually spoken of under the general term 

 irritability. 



This property, which is the most important of all, as it 

 is particularly the one which keeps the organism in a 

 proper relationship to its environment, is not always to be 

 observed or demonstrated 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 alterations of considerable 

 extent having been effected by the continuous 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 sur- 

 roundings 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. 



