IRRITABILITY; POLARITY AND CORRELATION 347 



Kinds of Stimuli. — There are many different kinds of stimuli 

 acting upon the plant. Among these are light, water, wind, 

 gravity, heat, the chemicals of the soil, electricity, pressure, etc. 

 Barnes (1910) attempted to classify these various stimuli as 

 mechanical, chemical, and ethereal, but since the nature of many 

 of these stimuli is as yet unknown, any kind of classification is 

 open to question. His classification may serve as well as any. 



Geotropism. — As an illustration of the reactions of the plant 

 to stimuli, the response to gravity will be discussed more fully. 

 It has long been noticed that plants grow in such a way that 

 stems go away from the earth, roots grow towards the earth, 

 and secondary branches and leaves grow out at various angles. 

 Roots, which go towards the pull of gravity, are said to be posi- 

 tively or progeotropic, stems are negatively or apogeotropic, while 

 leaves and branches which grow at right angles to the force of 

 gravity are plagiotropic. Certain roots and branches set them- 

 selves at a definite angle (other than 90°) to the force of gravity 

 or remain at a certain depth in the ground. Such organs are 

 diageotropic. 



That gravity is actually the cause of the position assumed by 

 these organs can be shown by (1) either neutralizing the effect of 

 gravity or by (2) replacing it with some other force. In the first 

 type of experiment, plants are placed upon a slowly revolving 

 wheel or disk called a clinostat. If a plant is revolved horizontally 

 on such a wheel rotating parallel to the long axis of the plant, the 

 roots and stems continue to grow in the direction in which they 

 were growing at the beginning of the experiment. The stems do 

 not turn up nor the roots down. Gravity here has a chance to 

 act on all sides of the stem equally, and the result is an equal 

 growth of all sides. If on the other hand a plant is laid horizon- 

 tally without rotating it, the stem will soon turn up and the roots 

 down in response to the influence of gravity. 



In Knight's experiment, the young plants are placed upon a 

 rapidly rotating disk so that the centrifugal force produced is 

 much greater than the pull of gravity. In this case the stems 

 turn toward the center of the wheel away from the pull of centrifu- 

 gal force, while the roots grow toward the circumference in the 

 direction of the pull. Here gravity is negligible as compared with 

 the much larger force exerted by the rotation. In both these 

 experiments, evidence is produced which leads to the unavoidable 



