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 behealthy. 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 
