232 THE PLANT WORLD. 



vanicum, Wallr., but X. glabratuiii (DC.) Britton and X. spi- 

 nosiini L. were also used. In the last named species the burs 

 have very thick indurated walls and the curvature is not well 

 marked, making them less satisfactory for study. Outside the 

 genus Xanthiniii there are no seeds which behave in a similar 

 manner, so far as can be ascertained, and no fruits of like 

 structure. 



A number of grasses have two seeds in a spikelet, the lower 

 seed usually being the larger and more vigorous. In the case of 

 oats careful tests show no difference in time of germination be- 

 tween the upper and lower seeds of a spikelet. But in the sand- 

 bur (Cenchrus trihuloides L.) the two seeds of the spikelet are 

 enclosed in a spiny covering which keeps them from readily 

 separating, and thus bear some superficial resemblance to the 

 cocklebur. In a trial in which sandburs were placed in a germi- 

 nating pan the upper seeds grew somewhat more slowly than 

 the lower ones, but in ten days practically all that were viable 

 had germinated, that is, 89 per cent, of the upper seeds grew 

 against 99 per cent, of the lower. These have been the most 

 favorable seeds for such a study, outside the genus Xanthium, 

 that have so far come under observation. 



In this study of the behavior of the paired seeds of cocklebur 

 it has not been possible to show that any structural or mechanical 

 difference exists, either in the seeds themselves or in their cover- 

 ings, which would account for the fact that while the lower seed 

 in a bur germinates normally the upper only grows after resting 

 one year or more. To be sure, it has been shown that the lower 

 is usually slightly larger and more vigorous, but this in itself could 

 not be the primary cause of difference in behavior. But, how- 

 ever the phenomenon is brought about, it is clear that by this very 

 serviceable adaptation the cocklebur has solved the problem of 

 giving each plantlet the utmost freedom for development, and 

 incidentally has increased the chances for the perpetuity of the 

 species. So far as the two seeds in each bur are concerned, it is 

 an instance of distribution of the plants by time rather than by 

 space, a seemingly unique character for the genus Xanthium. 



