THE NATURE OF IRRITABILITY 19 



The same applies to the actual irritability itself, for this is dependent 

 upon particular conditions, and changes as these alter. Below a certain 

 minimum or above a certain maximum temperature irritability ceases 

 to be manifested, and within these limits a change of temperature may 

 so alter the character of a particular irritability as, for example, to cause 

 a negatively heliotropic curvature to be produced instead of a positively 

 heliotropic one. 



Speaking generally, it is immaterial whether such a modification of 

 a particular irritability may take place under normal conditions, or be 

 induced by accidental and non-essential external stimuli. The character 

 of the work which a machine is capable of performing may be altered in 

 a similar manner. Thus a change in the position of the revolving cylinder 

 of a musical box causes a different response to be given, and a different tune 

 to be played when the handle is turned. 



It is only possible here to give a few examples of the varied and 

 reciprocal interactions of physiological stimuli. Thus mechanical injury 

 causes the primary leaf of Avena to lose for a time its sensibility to 

 heliotropic stimuli. Removal of the terminal bud of a fir-tree induces the 

 nearest lateral branch to assume a marked apogeotropic irritability and 

 become erect ; while in many rhizomes illumination causes diageotropic 

 irritability to be converted into geotropic. Again, in certain plants, in 

 order that the periodic movements dependent upon the alternation of 

 day and night may be continued, one of the necessary conditions is that 

 a certain inductive action of gravity shall be allowed to act upon the 

 plant. Moreover, it is very commonly the case that with an increasing 

 intensity of a chemical or other stimulus, the sensibility of the plant 

 diminishes in a corresponding ratio (Weber's law), or becomes modified or 

 completely altered : thus it is well known that many forms of irritability 

 may be weakened or inhibited by the action of ether or chloroform. 



Development and vital activity, together with the alterations of 

 irritability to which they may give rise, may be stimulated or modified by 

 factors of very special nature, although they are necessarily dependent upon 

 the maintenance of the essential general conditions : thus, when the young 

 shoots are killed by a spring frost, the more deeply situated reserve buds 

 are so stimulated that they are induced to develop and exchange their 

 passive mode of life for one of marked functional activity. Modifications 

 induced in this manner are of the utmost importance to the plant, and often 

 form the means by which a power is acquired of reacting in an appropriate 

 manner to new conditions. When we remember the varied nature of the 

 results which the plant needs, it is hardly surprising to find that such 

 modifications may either increase or diminish a pre-existent irritability, 

 or may even call new irritabilities into existence. Furthermore, it is well 

 known that the powerful stimulation of a particular sensitive organ, or any 



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