GROWTH AND MOVEMENT 



435 



is regained. By a larger dose irritability may be permanently abolished 

 (that is, it kills), while by a smaller dose it may become heightened. 

 Various narcotics act in a similar way. Substances that kill are usually 

 called poisons; really they are poisons only in certain doses. Their modes 

 of action are doubtless as different as the poisons themselves. 



In the following sections, the foregoing general principles will find specific 

 illustrations in the movements of locomotion, in the nastic and tropic curvatures 

 of various organs, in the displacement of leaves by motor organs, and in the effects 

 of stimuli upon form. It is important that the principles just set forth be constantly 

 referred to and kept in mind in reading these sections. 



3. MORPHOGENIC STIMULI 



The most general fashion in which various external agents affect 

 growth appears in the way they control the form of the body through local 

 alterations in the development of various parts. The varied and diffuse 

 stimuli are termed formative or morphogenic. The reactions to them are 

 extremely difficult to study because both stimuli and reactions are so 

 general, and particularly because experimental alteration of one factor 

 is almost certain to alter others to an unsuspected or an uncontrollable 

 extent; wherefore the analysis of the factors operating is rendered very 

 uncertain. It will be possible, therefore, to mention here only the 

 simpler and best attested examples. 



Light and growth. — It is well known that the rate of growth rises and 

 falls with the temperature, and since heat and light are both forms of 



Figs. 673, 674. — Graphs showing growth in millimeters in alternating periods of dark- 

 ness (shaded) and light: 673, sporangiophore of Mucor M uccdo, periods 15 minutes; 674, 

 rhizoids of MarcluitUia polymorplia, periods 20 minutes. — Based on data by Stameroff. 



radiant energy, it might be expected that the shorter and faster light 

 waves would also affect the rate of growth. This proves to be true. 

 In general the effect of light is to retard growth, particularly in elongating 



