DEVELOPMENT OF THE SEEDLING 189 



well known to you in house plants in windows. 

 The constancy of this turning suggests that 

 light-direction must determine it. To test this, 

 the influence of one-sided light must be removed. 

 This may be done either by placing the plant 

 in the dark (which, however, introduces abnor- 

 mal conditions), or else by making it revolve 

 so that one-sided light is made to neutralize 



its own effects. The latter has been done in 







Experiment 2. 



(1) Is light-direction a determinant of bend- 



ing of green leaves and stems ? An- 

 swer by observation of Experiment 2. 



(2) Is the process of turning (called helio- 



tropism) analogous to geotropism ? 



(3) For what good do leaves and stems 



turn toward the light ? 



(4) Do leaves and stems behave alike as 



to the positions they take relatively 

 to light-direction ? 



Materials. Either seedlings remaining in the boxes of 

 last week, or, since they can hardly grow enough in a week, 

 others grown on in ordinary boxes ; they are most useful if 

 the third and fourth leaves show. It is well to grow some 

 of them in the Wardian case, so the students can watch their 

 development. 



Pedagogics. This exercise (needing at least three two-hour 

 periods) is for further training in observation and morphology, 



