196 



ESSENTIALS OF BOTANY 



sometimes outgrowths from the ovary, sometimes from 

 the calyx, sometimes from an involucre. Their office is 

 to attach the fruit to the hair or fur of passing animals. 

 Often, as in sticktights (Fig. 147), the hooks are com- 

 paratively weak, but in other cases, as in the cockle- 

 bur (Fig. 147), and still more in the Martynia, the fruit of 



Fig. 147. Burs. 



A, sticktights; B, sticktights, two segments (magnified); 

 C, burdock; D, cockleburs. 



which in the green condition is much used for pickles, the 

 hooks are exceedingly strong. Cockleburs can hardly be 

 removed from the tails of horses and cattle, into which 

 they have become matted, without cutting out all the hairs 

 to which they are fastened. 



Why do bur-bearing plants often carry their fruit until 

 late winter or early spring ? 



