Page:Popular Science Monthly Volume 47.djvu/237

Rh 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 conditions of light, temperature, and moisture, are then to be considered 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 comfort 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 protoplasm 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 mechanism 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 necessities of the organism, but even in its highest form it must needs be