THE PAIRED SEEDS OF COCKLEBUR, 229 



Trials on a larger scale were made to ascertain the frequency 

 with which both seeds of a bur germinate at the same time. Of 

 137 burs placed in the germinating pans at one time, onh^ one 

 gave twin seedlings. Another year, 1787 burs produced 134 

 twin seedlings, the largest percentage obtained in any trial. 



It is unnecessary to give in detail the numerous observations 

 which finally led to the confirmation of the popular belief. From 

 the data obtained it is possible even to extend the statement. In 

 round numbers, one may assume that out of every hundred ap- 

 parently good cockleburs, one-fourth will not grow, owing to 

 various causes, especially the depredation of insect larvse and 

 failure to properly mature. Of the remaining three- fourths, about 

 five burs will produce two seedlings each, and about seventy 

 one seedling each. In the year following about thirty seedlings 

 will appear, in the third year about five, and two or three in subse- 



A B 



Fig. 39. Cocklebur. A. Vertical section showing one seed a little higher 

 than the other. B. One side a little more convex than the other. 



quent years. Thus it seems that the farmer is amply justified in 

 his dislike to these pestiferous weeds with their adaptation for 

 a prolonged occupancy of the soil. 



Following upon the establishment of the fact that the two 

 seeds in a bur behave dififerently under like conditions for ger- 

 mination, came a still more difficult task — the discovery of the 

 reason for it. Although the two seeds appear to be exact counter- 

 parts of each other, some difiference must really exist either in 

 the seeds or in the protecting bur. A careful microscopic exami- 

 nation of the seeds brought out no dififerences. The comparative 

 weight of the twin seeds was then obtained. Thirty-five perfect 

 burs were opened and the seeds accurately weighed on an ana- 



