THE GENERAL CHARACTERS OF SEEDS 485 



White-leaved Franseria (Franseria discolor Nuti..). 



Involucre with burs from a little less than one-quarter in. to 

 slightly more in length, with 2 achenes, each in a separate cell, 

 oblong, with 2 pointed spines usually incurved at apex and tapering 

 base ; several prominent ridges ; 3 or more tubercles, furrowed ; light 

 straw color, surface pubescent; achene slightly reticulated. 



Hooker's Franseria (Franseria hookeriana Nutt.). 



Involucre lanceolate or oval, two-fifths in. in length, with a promi- 

 nent conical spine and numerous, straight or recurved rigid spines, 

 variable in number, as long as the width of the bur ; surface wrin- 

 kled ; involucre with one elongated achene ; color yellowish, slightly 

 pubescent. 



Cocklebur (Xanthium canadense Mill.). 



Involucre containing 2 seeds; three-quarters to one in. or more 

 long, each in a separate cell, the lower placed further down in the 

 bur than the upper ; bur hard, woody, thick-walled, bearing numer- 

 ous hispid recurved spines, the 2 spines at the end thicker, heavier, 

 and incurved ; surface of achene blackish in color, thin-walled ; and 

 embryo slender with 2 long cotyledons and a thick caulicle; each 

 seed cavity connects with a channel, frequently may also show 

 styles connecting with this channel; odor of seed strong. 



A b * 



Fig. 359A Fig. 359B 



Fig. 359. Burs of Cockleburs. A. Common Cocklebur (Xanthium canadense), 

 with view of entire bur, and of cross section showing the pair of seeds in place. 

 B. Spiny Clotbur or Cocklebur (Xanthium spinosum). 



(After Hillman, Bull. Mich. Agr. Exp. Sta.) 



Cocklebur (Xanthium spinosum Kearney). 



Involucre one-third to one-half in. long, cylindraceous, obtuse, 

 armed with short prickles, inconspicuous, 2-beaked, or pointless, 

 occasionally a single straight spine ; bur oblong or somewhat flat- 

 tened ; surface sparingly covered with slender hooked prickles one- 



