Growth Movements 495 



of growth responses resulting in curvature or orientation of 

 members. As previously indicated, the movements of 

 plant members are now regarded as primarily of two types, 

 spontaneous (autonomic) and induced (paratonic). The 

 former are little understood. It is not always possible 

 to distinguish positively between the two types, or the 

 movement may be the result of conjoint (internal and 

 external) stimuli. In general, growth movement is a 

 fundamental requirement in the effective adjustment of 

 organisms to their environment. A study of the phenom- 

 ena is more important educationally in liberalizing our 

 views of plant relations than of any direct assistance in 

 special problems of plant production. 



300. Stimulus and response. The relations of or- 

 ganisms to growth factors have been considered, but it is 

 necessary to refer again to the environmental forces which 

 condition plant activity. When the environment is 

 favorable the plant is regarded as exhibiting the condition 

 of tone ; and the effect of each factor or of the various 

 factors severally is a tonic influence. The factors con- 

 cerned are those essential in growth, especially oxygen, mois- 

 ture, food-supply, light, and heat. They are sometimes 

 known as the formal growth conditions. Some of these, 

 and likewise other environmental factors, may act not as 

 tonic influences but as special stimuli releasing growth 

 responses, that is, movements. 



i The plant is not merely a complex mechanism ; it is, 

 when in a condition of tone, a source of readily releasable 

 energy, growth energy. It requires a stimulus to make 

 this energy manifest, but it appears unnecessary that the 

 stimulus should impart force. As so often pointed out, 



