CHAPTER V. 



IRRITABILITY. 



40. Nature of Irritability. — The term irritability designates 

 that property of plants by which they respond to certain influ- 

 ences known as stimuli. The stimuli may be either internal 

 forces set in operation by metabolic activity or external influ- 

 ences, such as gravity, light, temperature, electricity, moisture, 

 and mechanical contact. The plant may react in two ways, 

 first, by changes in the structure, form, and size of its organs ; 

 second, by motion or change in position of its organs or of the 

 protoplasmic bodies in its cells. The reactions of the first class 

 concern growth ; those of the second class result in placing 

 plant or cell organs in certain positions relative to the direction 

 of the stimulus. Thus a plant grown in darkness develops its 

 stems and leaves in quite different form and structure from 

 one grown in the open air (Fig. 6^). Light, then, affects the 

 structure and form of plants by what is known as its formative 

 or tonic influence. (See Chapter VI.) Light also causes shoots 

 to bend toward its source in such manner as to place their 

 axes parallel to the light-rays. This is termed its directive in- 

 fluence. A stimulus may give rise to reactions of both kinds 

 at the same time, and they cannot always be easily and dis- 

 tinctly separated by experiments. 



41. Perceptive and Motor Zones. — The action of an external 

 stimulus on one part of a plant does not necessarily cause a 

 movement in that part, but the impulse may be transmitted to 

 a region more or less distant. Thus, for instance, a touch on the 



leaf-blade of a Mimosa (Sensitive-plant) causes no contraction 



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