Various Physical Factors 347 



that if stem bases were enclosed in plaster and thus relieved of mechani- 

 cal strain they produced less mechanical tissue. 



Ball (1904), however, repeated Hegler's experiments, using pulleys 

 and carefully comparing stretched plants with their controls, but found 

 no difference between them in structure or tensile strength. Hibbard 

 (1907) confirmed Ball. Still later Bordner (1909) studied the problem 

 again and confirmed Hegler's results, using similar material. In stretched 

 plants the amount of vascular tissue and the tensile strength were in- 



Fig. 16-1. Vicia faba. Outer portions of 

 transverse sections through the second 

 internode. a, grown in darkness without 

 shaking; b, grown in light; c, grown in 

 darkness and shaken. Mechanical stimu- 

 lation has much the same effect on 

 growth as does light. ( From Bunning. ) 



creased. He found that no effects were produced unless the plants were 

 growing, a fact that may help explain the conflicting results of these 

 various workers. 



Flaskamper (1910) and others subjected flower and fruit stalks to ad- 

 ditional weighting but found no change in histological structure. Wieders- 

 heim ( 1903 ) hung weights on branches of a weeping variety of beech 

 and reported that these grew less rapidly and had shorter cells but formed 

 no additional vascular tissue. 



In all this early work on mechanical factors the suggestion was natural 



