82 



Trigger-Hairs of the Thistle Flower. 



The common pasture thistle {Cnictis altissimiis, Willd.) is a 

 good subject for the study of sensitive stamens. • It is needless to 

 remind the readers of the BULLETIN that the stamens in the 

 thistle flower are five in number and are united by their anthers 

 so as to form a tube, which surrounds the upper portion of the 

 slender style. The filaments are free from each other for some 

 distance below the anther-ring, and in these parts is the seat of 

 the characteristic movements. 



A freshly opened flower has the tip of the style within the 

 anther- ring. When such a blossom is touched by a pencil point 

 or needle, the anthers are drawn down and the upper end of the 

 style may be brought' through the ring of anthers along with a 

 quantity of pollen adhering to the spinose surface. If, after a 

 few minutes, the flower is touched again, the motion is renewed 

 and the ring is pulled still further down. By irritating the an- 

 droecium upon one of the filaments and then upon another oppo- 

 site it, the blossom may be caused to sway from one side to the 

 other. In short, those slender filaments are sensitive, and when 



touched will contract and thereby pull the united anthers down- 

 ward. 



•A 



The peculiarities of the movements have led to a microscopic 

 examination of the filaments. They consist of colorless cells, two 

 or three times as long as broad, placed end to end and surround- 

 ing a small central bundle of six to ten closely coiled spiral ves- 

 sels. In transverse section the filament is nearly triangular, the 

 broader .side being placed nearest the style. A portion of a 

 young filament is shown at a, in the engraving. 



Many hairs are found upon the surface of the older filaments, 

 and exhibit a peculiar structure. Each trichome consists of two 

 nearly parallel cells, which extend side by side to nearly the end 

 of the outgrowth. Usually, one of these cells takes the lead, and 

 in an old hair it forms the end, which is somewhat enlarged. 

 The other cell is closely applied to the first and a Uttle below the 

 extremity of the trichome. There is a hyaline outer layer com- 

 mon to the two cells. 



It is not difficult to determine the origin and development of 



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