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 format vue or morpho < genie. 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 Mucedo, periods 15 minutes; 674, 
rhizoids of Marchantia polymorpha, 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 
