IRRITABILITY 



23 



forms of irritability are found to reside in the 

 same organ, as for example, in tendrils. In 

 such instances, however, the excessive num- 

 ber of the forms of irritability have beeen de- 

 veloped to meet special ecological conditions, 

 bearing upon both the nutritive and repro- 

 ductive processes either directly or indirectly. 

 Furthermore, the organs of the shoot may 

 also acquire the power of special reactions to 

 internal forces or stimuli, such for example, 

 as the carpotropic movements. 



In a consideration of the localization and 

 distribution of the property of irritability, 

 attention is to be called to the fact, that the 

 conditions concerned in the nutritive processes 

 of the shoot show an invariably wide diffu- 

 sion. Carbon dioxide exists everywhere in 

 the atmosphere in uniform proportions and 

 bathes every part of the shoot. Sunlight is 

 bounded only by the horizon line and may 

 reach any surface of the shoot in diffuse form. 

 The chlorophyll processes may then be carried 

 on by the sub-epidermal tissues in any por- 

 tion of the shoot, and as a consequence, a 

 greater proportion of the peripheral proto- 

 plasm of the shoot has developed an irrita- 

 bility to sunlight, although it may not al- 

 w^ays be manifested by organic or external 

 movement, or other response. 



The researches of Rothert have shown that 



Distribution 

 of nutritive 

 factors 



