IRRITABILITY AND MOVEMENT IN PLANTS. 227 



Protoplasm, the physical basis of animal and plant life, has 

 among other characteristics that of irritability to several classes 

 of stimuli furnished by its environment. It reacts to these stimuli 

 by adjustments, many of which are accomplished by motion or 

 contraction, and to others by metabolic changes. What agencies 

 have been potent in the development of this primal irritability of 

 protoplasm, through reflex action, into sensorial reaction in the 

 animal, and into the various forms of specific irritability in the 

 plant ? 



It is unquestionable that the paramount necessity for every 

 organism is that of self-preservation. To obtain food, avoid 

 injury, and secure the proper degree of the environmental con- 

 ditions of light, temperature, and moisture, are then to be consid- 

 ered as the fundamental necessities of every organism.* Animals 

 are organisms in which destructive metabolism prevails, in which 

 more energy derived from complex foods is dissipated than is 

 conserved. Connected with and underlying this condition of the 

 metabolic balance is the fact that animals have steadily developed 

 toward motile forms. In the accomplishment of the conditions 

 of life, motion has become to them an indispensable function. 

 The necessity for the ability to direct the locomotory movements 

 in the avoidance of danger and the attainment of food and com- 

 fort has led to the development of irritability into the forms of 

 sensorial action. Plants, on the other hand, are organisms in 

 which constructive metabolism prevails, in which more energy is 

 conserved than is consumed in the performance of the necessary 

 work. Underlying this state of the metabolic balance is the fact 

 that plants have steadily moved along a line toward fixed forms. 

 Consequently irritability has been developed into forms which 

 would be of service to the plant in securing food and protection 

 without moving from place to place. Not only has the proto- 

 plasm of the plant developed an irritability to different qualities 

 of the stimuli to which animal protoplasm responds, but it also 

 reacts to certain forces to which the animal is inert, by a mechan- 

 ism different in every essential from that of the animal. These 

 two lines of development of the primal irritability of protoplasm 

 by reason of the metabolic activity and other conditions attendant 

 on each are so widely divergent that great care must be exercised 

 in the comparison of the higher forms of sensibility exhibited by 

 the plant and the animal. In the animal the higher form is that 

 of sensorial reaction with its vast range of usefulness. In the 

 plant this power has developed with equal facility for the necessi- 

 ties of the organism, but even in its highest form it must needs be 



* Arthur, Special Senses of Plants. Proceedings of the Indiana Academy of Sciences, 

 1893. 



