230 THE PLANT WORLD. 



lytical balance. The thirty-five heavier seeds of each pair gave an 

 average weight of 65 milligrams, while the thirty-five lighter 

 seeds gave an average weight of 46 milligrams, a difference of 

 19 milligrams. This promising result appeared to be of no 

 significance, however, when the seeds were placed in the germi- 

 nating pan, for the two lots showed no difiference in rate of 

 germination. 



As study of the paired seeds revealed no recognizable features 

 to account for the difiference of behavior, attention was turned to 

 the enclosing bur. It was thought that perhaps one side of the 

 bur was not pervious to water until partly decayed. To test 

 this, burs were placed in chemical solutions and colored liquids. 

 But on cutting them open, however short the immersion, both 

 cavities of the bur were found equally and abundantly afifected. 

 A little further examination showed that the opening in the apex 

 of each half of the bur, through which the style once protruded, 

 was still in the ripe fruit an opening large enough to admit a 

 needle, and that, furthermore, a considerable channel existed in 

 the base of each half, closed only with loose fibrous material. 

 This structure permits liquids to penetrate through both ends of 

 the bur without hindrance, and to pass into both seed cavities with 

 equal readiness. 



At the beginning of the sixth year of the study an important 

 discovery was made. In dissecting the cockleburs it was necessary 

 to make longitudinal sections, and in this way attention was at- 

 tracted to a difiference in position of the two seeds ; one seed is 

 placed a little higher in the bur than the other, see fig. i. This 

 slight but significant difiference seems to have escaped the illus- 

 trators, for a considerable search through standard and miscellane- 

 ous botanical books has failed to bring to light any cuts showing 

 the seeds otherwise than exactly side by side. Further examination 

 made it possible to tell without dissection on which side of the 

 bur the higher or the lower seed lay. The unsymmetrical posi- 

 tion of the seeds causes a slight curvature of the bur, see fig. 2, 

 and the lower seed lies on the inner or less convex side, while the 

 higher seed lies on the more convex side. Here was the key to 

 the situation. It was now possible without injury to the burs to 



