228 THE POPULAR SCIENCE MONTHLY. 



termed irritability, although it has attained a degree of specializa- 

 tion in which its delicacy and usefulness are equal to those of sen- 

 sorial 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 re- 

 sponse 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 as- 

 sumed 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 pri- 

 mary 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 aerial organs is that they 

 assume a position in which 

 their surfaces will be exposed 



Fig. 1,-Downward Curvature of a Primary to the g^ig]^ a t an angle 



KOOT OF PlSUM WHICH HAS BEEN PLACED 



in a Horizontal Position. most advantageous for the 



performance of their func- 

 tions 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 sur- 

 faces 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 



