Power to Adjust Parts to Surroundings 251 



whether a symmetrical cone of horizontally-spread branches ra- 

 diating from a central main stem, as in the Firs or the Spruces, 

 or a great urn of up-and-outcurved branches, as in the Maples 

 and Elms, or in any of the intermediate shapes; and the reader 

 should learn to visualize all of the main trunks and branches as 

 thus developing in touch with gravitation and largely under its 

 guidance. This applies, however, only to the main structures; 

 the smaller branches and most of the minor parts are more or 

 less controlled by other kinds of stimuli which determine the 

 final details of form; and this is especially the case with roots. 

 The fact that geotropism is thus ever tending to hold the plant 

 to a certain upright symmetrical form explains why any one- 

 sided turning in response to other stimuli, is of Imiited amount, 

 and why the plant always tends to recover its former upright and 

 symmetrical position in case it is disturbed. 



Some minor tropisms. — These include, — thermotropism, a turn- 

 ing towards warmth, rather rare: traumatropism, the turning 

 of roots away from an external irritation or injury: rheotropism, 

 a turning against a water current, which, however, has been 

 shown to be only a special phase of thigmotropism : electro- 

 tropism, a certain adjustment to mild electric currents; and some 

 others of lesser importance. The case of rheotropism, by the 

 way, illustrates a confusion of stimuli, the root apparently mistak- 

 ing the pressure of the flowing water for that of some hard ob- 

 ject in the soil. The case of electrotropism, involving response 

 to an influence to which the plant is never subjected in nature 

 and to which it cannot have become adaptively sensitive, illus- 

 trates the same thing, or else, perhaps, an accidental release of the 

 motor-mechanism after the manner already described for the 

 Sensitive Plant. And the occasional responses found in plants 

 to other stimuli new to them (e. g. to X-rays, radium emana- 

 tions), are hkewise due without doubt to confusion of stimuli, or 

 accidental release. 



Thus far we have considered for the most part only cases in 



