196 « ELEMENTS 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 
Fic. 147. Burs. 
A, sticktights; B, sticktights, two segments (magnified) ; 
C, burdock; D, cockleburs. 
which in the gteen condition is much used for pickles, the 
hooks are exceedingly strong. Cockleburs ean hardly be 
removed from the tails of horses and eattle, into whieh 
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 ? 
