Distribution.--A very common weed in dry or moist soil throughout eastern North America. Extermination.-Apply the same method as to the preceding species. ZYGOPHYLLACEAE, CALTROP FAMILY. A small family. Caltrop (Tribulus terrestris L.). Description. The caltrop is a hairy, procumbent annual, branching from the base, producing a stem which is a foot or more long, branches bear numerous small, compound leaves with short peduncles and small stipules at the base; each compound leaf has 4-8 pairs of short-stalked leaves; small, yellow, axillary flowers about 11⁄2 in. across with peduncle much shorter than leaves; fruit very spiny and divided into two nearly equal parts, each part consists of 2 long spines, 2 shorter and a row of very short ones, forming a crest on the back; 5-angled, spiny fruit splits into 3-5 divisions. Distribution.-Introduced from the Old World and occurs from the Atlantic states to Nebraska and Kansas; in Iowa has been found only on Muscatine Island. Extermination.-Caltrop is disseminated by wool, and hence the waste of woolen mills should not be thrown in fields. The weed is easily destroyed by cultivation. Fig. 118. Caltrop, Ground Bur-nut (Tribulus terrestris). Sandy scil, waste places, Muscatine Island. (Drawing by Charlotte M. King.) Many of the plants of this family contain an irritating milky juice. Few are of economic importance. The poinsettia commonly cultivated in greenhouses, snow-on-the-mountain in gardens, and castor-oil bean belong to this family. Three-seeded Mercury (Acalypha virginica L.). Description. A smoothish or hairy annual from 1-2 ft. high, turning purple especially in the autumn; leaves ovate or oblongovate, sparingly serrate, long-petioled; sterile spike, few-flowered, pistillate flowers 1-3 at the base of staminate peduncle surrounded by a large leaf-like bract; capsule 3-lobed, subglobular, 2-valved carpels. Distribution.-From Nova Scotia to Texas and northward to Minnesota. Common everywhere in Iowa along roadsides and in fields. Especially noticeable in the fall on account of the purple bracts. Extermination. Three-seeded Mercury is not a difficult weed to exterminate. The small, reddish, striate seeds are expelled from the plant to some little distance in a manner similar to the dispersal of the castor-oil bean. Thorough cultivation by preventing the formation of seed will eradicate the weed. |