The mechanism of seed-dispersal in Polygonum virginianum 



Howard Sprague Reed and Isadore Smoot 



The achenes of Polygonum virginianum Linnaeus have a par- 

 ticularly efficient mode of dispersal. The long two-parted styles 

 are persistent and become lignified during the process of develop- 

 ment. If, after maturity, these projecting styles are struck by any 

 object with sufficient impact, the achenes are thrown off with force 

 enough to carry them a distance of three or four meters. 



During windy weather the tall, slender racemes are blown 

 against each other and against the strong stems of plants like 

 Phytolacca and Sambucus, 

 with which this species of 

 Polygonum is associated. 

 The impact thus produced 

 is sufficient to throw off the 

 mature achenes without the 

 agency of animals. Never- 

 theless, animals are very 

 efficient means of dissemin- 

 ation, for, if they do touch 

 the trigger-like styles, the 

 achenes are thrown with 

 some force against the body 

 of the animal, where they 

 cling, the sharp, reflexed . . . 



Fig. t. A mature achene of Polygonum virgini- 



anum. 



points of the style (see fig. 



i) becoming fixed in the 



hair, or fleece, and they may thus be carried some distance. 



All botanical text-books give examples of fruits and seeds 

 which are forcibly ejected when ripe. In most cases the force is 

 due to some property of the ripened wall of the gynoecium. The 

 ejection of the achenes of Polygonum virginianum is unique in that 

 the requisite force is derived from a tension in the pedicel. 



* Contribution No. 9 from the Botanical Laboratory of the University of Missouri. 



377 



