INTRODUCTION 13 



Hydrotropism is a movement of the roots or root- 

 hairs towards the moisture in soil, and, according as 

 it varies, the direction of growth varies. 



There are also movements which are due to the 

 alternation of day and night or alternation of tem- 

 perature. These are called nyctitropic movements or 

 " sleep movements." In this case the leaflets of certain 

 plants close up or assume a vertical position at night, 

 as in the Woodsorrel (see vol. ii. Fig. 32, p. 168), 

 They are possibly thus protected from frost or cold. 



Flowers close up during the night and in wet 

 weather to protect the essential organs, or stamens 

 and pistil, as well as the pollen and honey, from cold. 



The irritability of the tentacles of the insectivorous 

 plants is another form of movement connected with 

 their special mode of nutrition (see under Drosera, 

 vol. ii). 



Each organ of the plant exhibits different degrees 

 of irritability or response to stimuli in relation to 

 nutrition or other factors. 



The root responds to the stimuli of gravity, light, 

 temperature, moisture, water, nutrient solutions, 

 electricity, injury. 



The shoot responds to the stimuli of light, contact 

 in the case of climbing plants, and internal reactions 

 such as carpotropic movements and to injury and 

 electricity. 



The leaves respond to light or heat, or generally 

 to the sunlight in relation to nutrition. 



The flower responds to the stimulus of heat in the 



