DISPERSION. 



299 



employed in ancient warfare to harass the enemy's 

 cavalry. One of these kind of caltrops (Tributes 

 terrestris) is widely diffused, probably on account 

 of the facility with which the fruits are transported 



Fig. 54. — Caltrops, or fruits of Tribulus terrestris. 



in the wool of animals. They have an elegant, 

 symmetrical, star-like form, and the spines are very 

 sharp and rigid. Another, but less complex, fruit 

 (Pedalium mttrex) has its dispersion provided for in a 

 similar manner. The name of caltrops has also been 



Fig- 55- — Fruits of Pedalium murex. 



given to one of our indigenous plants, called also "star- 

 thistle," on account of the sharp spines of its woody 

 involucre (Centaurea calcitrapa). More efficient still 

 are' the recurved hooks with which some of the 

 spines of fruits are terminated. To a limited extent 



