234 PLANT PHYSIOLOGY 



Our consideration of geotropism has shown that gravita- 

 tion acts not mechanically, but as a signal stimulus, that it is 

 perceived at one place while response takes place at another, 

 that responses can be positive, negative, or intermediate to the 

 same stimulus, and that the responses are such as to bring the 

 parts as a rule into positions favorable for their functions, though 

 under exceptional conditions they may bring the parts into 

 unfavorable, or even fatal, positions. Since these features are 

 true for one form of irritability, they may be true for others, a 

 matter whose importance will be evident on further study. 



(b) Phototropism. 



The most familiar example, no doubt, of irritable responses 

 of plants to external influences is the turning of green tissues 

 towards light (phototropism or heliotropism). A logical begin- 

 ning of its study is this problem : 



What are the jacts in the turning 0} green parts towards light? 



Experiment. In the two compartments of a demonstration 

 heliotropism chamber, place plants of Garden Nasturtium or other 

 slender-petioled growing plant (or young seedlings), one in fixed posi- 

 tion and the other revolving with axis vertical on a klinostat. Give 

 strong one-sided light, and observe the resultant leaf-and-stem posi- 

 tions. 



Heliotropism Chambers. A variety of forms exist, adapted to special 

 phases of the study, all, however, being designed to keep the plant under 

 healthful conditions in a dark chamber to which light can be admitted under 

 control (compare MacDougal, 129, who gives a form ventilated by aid 

 of an aspirator, Detmer, 468, and the color chamber of Sachs in his Gesam- 

 melte Abhandlungen, 1, 299). For demonstration purposes, however, I 

 have found very excellent a box, black inside, white outside, divided by a 

 partition into two chambers each about 20X20 cm. square and 40 cm. high, 

 open on one side. The light is then wholly one-sided, and can be further 

 regulated as to amount and direction by a covering of black-and-white paper 

 glued in place over the open side. A handle on top facilitates its ready trans- 

 port. 



The student should carefully note in this experiment the dif- 

 ferences in the positions of stems and leaves, which will suggest 

 in turn this question: 



What response is made by roots to light? 



