THE AMERICAN BOTANIST 149 



as ever, single plants spreading over a circle nearly six feet 

 in diameter. A remarkable instance of the tenacity with 

 which this species clings to life was found in one plant which 

 was overlooked in the spring of 1920 and which did not get 

 back into the ground again until May 1921. After nineteen 

 months without soil or water it grew promptly and has since 

 flowered profusely. A further proof of the hardiness of the 

 four-o'clock is found in the fact that some plants left out in 

 the ground without protection during the recent mild winter, 

 came through unharmed in the latitude of Chicago. 



Tribulus Becomes a Tribueation. — In the days when 

 knighthood was in blossom, some ingenious blacksmith inven- 

 ted a devilish instrument called a caltrop which consisted of 

 four spikes so arranged that no matter in what position placed, 

 one of them projected upward. When things looked darker 

 than usual for the knights, they often saved their hardware 

 by strewing their retreat with caltrops. These penetrating the 

 feet of their foe's horses effectually held them in check. As 

 in so many cases, however, nature seems to have thought of 

 the scheme first for the species of Tribulus are equipped with 

 fruits that are pretty good likenesses to caltrops.. The plants 

 are originally from Europe but they have escaped to America 

 and thrive especially in our West where they strew the ground 

 in just the right position to interfere with the automobile traf- 

 fic. According to the United States Department of Agricul- 

 ture, they produce as effective punctures as ever a man-made 

 caltrop did. The plants lie flat on the ground and are not 

 easily eradicated by the usual means. Tourists call the plant 

 "devil's bur", "puncture vine" and "automobile weed." It was 

 formerly known as "ground-bur." 



Peant Hairs oe Honey Locust. — W. B. McDougal has 

 found that root-hairs of honey locust {Gleditsia triacanthos) 



