Reed and Smoot: Polygonum virginianum 



385 



quent drying. Several mature racemes were placed in a moist 

 chamber with a saturated atmosphere and allowed to remain there 

 for four days, during which time the achenes remained attached. 

 The racemes were then removed and placed in an oven, heated to 



Fig. 7. Longitudinal section of thin-walled pith-cells which form the cushion (cf. 

 figures 2 and 3, /). 



75 degrees C. After seven hours they were again examined, but 

 the effect of the drying had not been sufficient to detach any of 

 the achenes. 



The phenomenon of dispersal awaits an impact strong enough 

 to break the fibrovascular cylinder and release the compressed 

 cushion of cells. It is principally dependent on three things — a 

 specially developed u separation-layer "; the existence of a longi- 

 tudinal tension between the pith-column and the investing fibro- 

 vascular cylinder ; and a blow on the achene analogous to the 

 pull on the trigger which discharges a gun. 



University of Missouri, 

 Columbia, Missouri. 



