378 Reed and Smoot : Polygonum virginianum 



Hildebrand ('73) mentions this plant as one whose achenes 

 are transported by animals, but appears not to have noticed their 

 forcible expulsion. He mentions only the hooked styles which 

 fasten the achenes to the hair, or fleece, of animals. 



Kerner ('95) described it as a " catapult fruit/' He perceived 

 the separation-layer in the pedicel, but gave no satisfactory ex- 

 planation of the manner in which ejection was accomplished. His 

 account (2 : 842) might lead one to think that the tension was 

 produced by the cells of the cortical parenchyma. We have not 

 found it to be so. 



The devices of the plant for dispersing its fruits have been noted 

 by Beal ('98), but no explanation was given. 



This paper relates the results of a histological study of the 

 tissues in question. Since the achenes fly off with a force entirely 

 disproportionate to that which strikes them, it may be assumed 

 that there is some tension existing in the pedicels which, when 

 released, throws the achenes from the plant. It has been sup- 

 posed that the existing tension was given by changes in the tissues 

 of the pedicel as they became dry. The attempt was made to dis- 

 cover whether changes might be found in the structure or arrange- 

 ment of these tissues sufficient to account for the phenomenon of 

 dispersal. 



The material studied was collected near Columbia, Missouri, 

 in the autumn of 1904, and during the summer and autumn of 

 1905. All stages were obtained, from the first formation of the 

 raceme to the mature achene. The killing fluids used were 

 mercuric-bichloride in 95 percent, alcohol, Mann's picro-corrosive 

 fluid, chrom acetic acid, and Worcester's killing fluid. The older 

 material was embedded in celloidin and the younger in paraffin. 

 The stains used were fuchsin- iodine-green, iron-alum-haematoxylin 

 and safranin, and Delafield's haematoxylin. 



P.jvirginianum constitutes the only member of the subgenus 

 Tovara, according to Small ('95). The plants are herbaceous 

 annuals, somewhat woody at the base. When the species of this 

 genus are arranged according to their histological structure, P. 



virginianum appears to stand about midway in the series. The 

 epidermis, like that of nearly all the other species, consists of a single 

 row of cells. The hypodermal tissues are not so well developed 



