Minnesota Plant Life. 



405 



Ragweeds and cockleburs. The ragweed family, with six 

 or seven native species, includes the ragweeds, cockleburs and 

 marsh-elders. The cocklebur, of which two varieties occur in 

 Minnesota, is known by the conversion of the involucre of each 

 pistillate head into a two-pronged, many-hooked bur in which 

 two fruits are enclosed. A striking peculiarity of the cockle- 

 bur is that one of its two fruits will germinate the first season, 

 while the other ordinarily lies dormant for a year. The marsh- 

 elder is, in Minnesota, a tall roadside weed with leaves shaped 

 like those of the cocklebur. The panicled heads of green stam- 



inate flowers are intermixed 

 with small pistillate flowers 

 without corollas. The rag- 

 weeds, of which three species 

 are common in Minnesota, are 

 also known as "hay-fever 

 plants," because their copious 

 pollen-spores produced in the 

 autumn will, if inhaled, some- 

 times germinate in the nostrils 

 and the little thread-like male 

 plants will then irritate the 

 nasal membranes of persons 

 subject to the disease. The 

 tall ragweed, which sometimes 

 grows fifteen feet in height in 

 low ground along roadsides, 

 has three-divided leaves and 

 numerous slender racemes of heads. The other ragweeds have 

 pinnately divided or compounded leaves, and are a foot or two 

 tall. 



Sunflowers and their relatives. The sunflower family has 

 about 200 native species. Here are classified the ironweeds, 

 herbs of moist soil with willow-shaped leaves and purplish small 

 heads, arranged in flat-topped clusters ; also the thoroughworts, 

 distinguished from the ironweeds, which in outward appearance 

 they much resemble, by the pappus. In thoroughworts this is 

 composed of numerous slender bristles, while in the ironweeds 

 it is double, the inner whorls alone being constituted of bristles, 



Fig. 199. 



Ragweed. After Britten and 

 Brown. 



