WEEDS OF THE THISTLE FAMILY. 



155 



feet high with the bracts of the involucre brownish-purple, tipped 



with spreading awns. 



The perennial roots of all 



St "-^9k ^ these are stout and fibrous, and 



1 eac h autumn are filled with a suf- 

 ficient supply of nourishment to 

 give the stalk of the ensuing year 

 a good start in life. They radiate 

 in all directions from the base of 

 the stem, spreading over an area 

 of several square yards and pene- 

 trating the soil in search of 

 moisture to such a depth as to 

 render abortive any attempt of 

 man to pull the plant up bodily, 

 roots and all. The leaves are so 

 innutritions that none of the 

 higher animals, not even sheep, 

 will feed upon them. 



The only insect enemies of the 

 Fig. 114. Western iron-weed. iron-weeds, so far as noticed, are 



the margined and black blister beetles* which attack the leaves 

 when other food is scarce, and a small gall-fly whose larvae feed 

 upon the juices of the flowering branches. They are also preyed 

 upon at times by the leaf and downy mildews and by several rusts, 

 but none of these serve to retard their growth to any great extent. 

 Many species of bumble-bees and butterflies visit the blossoms in 

 search of nectar and pollen, and thus aid materially in their fertil- 

 ization. The flowers in each head number, on the average, 25, 

 each of which produces a single seed. On one specimen of medium 

 size were counted 743 heads, so that 18,575 seeds, each capable of 

 becoming a fully developed iron-weed, were borne by that plant 

 alone, and the majority produce as many, or more. To secure a 

 broad dissemination each of these seeds bears at maturity a tuft or 

 pappus of light brown bristles, and by its aid the seed may be 

 wafted by the wind miles away from the parent plant. Again, as 

 the iron-weed grows in greatest luxuriance in the lowland pastures 

 near small streams, many of the seeds fall upon the water and are 

 borne onward till they lodge against some bank or are buried in 

 the sediment deposited by an overflow ; places well suited for their 

 future growth. In these ways the weed is continually spreading 



*Epicauta marginata Fab. and E. pennsylvanica DeG. 



