Volume 9 Number lo 



The Plant World 



31 f^a%R}ix\t nf Popular ^otanp 

 OCTOBER, 1906 



THE PAIRED SEEDS OF COCKLEBUR. 



By Professor J. C. Arthur, 

 Purdue University. 



The cocklebur (Xanfhiiim) is a common native weed through- 

 out the central and eastern United States, especially along sandy 

 river bottoms and in corn fields. The seeds are enclosed in a 

 hard, spiny covering, two in each bur. The spines undoubtedly 

 assist in the distribution of the seeds by clinging to the rough 

 coats of animals. Unlike many other seed pods, they do not open 

 to discharge the seeds, either at maturity or by subsequent 

 changes of moisture or temperature. Not only is the bur inde- 

 hiscent, but it does not readily disintegrate, and after a number 

 of years in the soil may appear little changed. From this hard, 

 unyielding bur the young plantlet frees itself at the time of 

 germination by first pushing out the radicle and then pulling out 

 the seed leaves, while the bur retains its usual appearance. 



There is quite a common belief among farmers that one of the 

 seeds in a cocklebur germinates the first year after maturity, and 

 the other seed not until the year following, thus accounting for the 

 persistency of the weed in cultivated fields. The truth in this 

 theory was not tested, I believe, until a few years ago, when the 

 writer took up the matter and made some preliminary trials which 

 led to a study of the subject extending through seven years, and 

 which established the essential correctness of the farmers' opinion. 



