Various Physical Factors 353 



Many experiments have been conducted, beginning with Knight 

 (1811), on the effect of continually swaying plants as they grow. The 

 swaying was done by clockwork and pendulum, water wheel, or motor. 

 Observers are generally agreed that the cross section of the axis tends to 

 be elliptical under these conditions, with its wider dimension in the 

 plane of sway (Fig. 16-6), and that more vascular and mechanical tissue 

 is developed than in the controls (Rasdorsky, 1925; Burns, 1920). 



It may be objected that in these swaying experiments conditions are 

 so abnormal that results may be due to other factors than purely mechani- 

 cal ones. The experiments of M. R. Jacobs (1954) are significant here. He 

 supported the trunk of a young pine tree with guy wires attached about 



Fig. 16-6. Effect of swaying on sunflower stem, cross sections. At left, control. At 

 right, stem swayed for 3 weeks in the vertical plane of the figure. ( From Rasdorsky. ) 



20 feet from the ground so that lateral movement below this point was 

 prevented although the trunk above the point of attachment was sub- 

 ject to ordinary wind sway. This portion increased normally in thickness 

 but the unswayed lower portion grew much less rapidly than the upper 

 or than the unsupported controls. When the guy wires were removed, 

 this part grew rapidly in thickness until its normal diameter had been 

 attained. Evidently in such cases swaying stimulates cambial activity. 

 This may be a factor in producing the relatively slender trunks of trees 

 in a forest as compared with those grown in the open. 



A somewhat similar case has been reported by Venning (1949) for 

 celery, an herbaceous plant. One series of seedlings was grown in con- 



