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 
2 “. A 
Fic. 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? 
