Page:Popular Science Monthly Volume 47.djvu/238

 termed irritability, although it has attained a degree of specialization in which its delicacy and usefulness are equal to those of sensorial action, and may surpass them in some instances.

Among the more marked forms of irritability shown by the plant may be mentioned those by which it reacts to gravity, radiant energy (heat, light, and electricity), shock, and metabolic action. A clear conception of the character of the reactions of plants to these stimuli may be attained by a consideration of those shown toward gravity, light, and contact of solid bodies.

Since the plant is a fixed organism and can not move in search of food, it is essential that its roots thoroughly penetrate the soil of the locality in which it is found, in such manner as to place its absorbing surfaces (root hairs) in contact with whatever nutrient solutions the substratum may contain. The necessity for such penetration of the soil has led to the development of irritability to gravity in the protoplasm of the roots. Primary roots in response to the stimulus of gravity tend to place themselves in a position with their axes lying parallel to the force of gravity and the tips pointing vertically downward. But if all the roots assumed this position they would depend from the stem in a compact mass in a manner not advantageous to the plant. The secondary or smaller roots, however, react to gravity in such manner that they place their axes at right angles to the line of force, thus securing a penetration of the soil in a second direction. These forms of reaction to gravity are also exhibited by other organs of the plant but do not occur to any extent among animals. The manner in which a primary root curves to place its axis in a vertical position may be seen in Fig. 1, and that of a culm of grass to become erect in Fig. 2.

An important requirement of aërial organs is that they assume a position in which their surfaces will be exposed to the sunlight at an angle most advantageous for the performance of their functions of the formation of food and transpiration. To meet this need, those portions exposed to the light have acquired a specific manner of response to the light by which some place their surfaces parallel and others at right angles to the direction of the rays. Gravity acts continually and invariably in one direction, and with a constant force. Consequently the movements of