IOWA ACADEMY OF SCIENCES 21 9 
will be seen to move from one side and back in an irregu- 
lar manner. A contraction of the filaments takes place; 
and it is this contraction that causes the pollen to be 
pushed out. This motion takes place when the insect 
seeks the nectar which is secreted by a ring surrounding 
Fig. 12. Pollination of thistle. 
a , The outer glandular bract; a 1 , dorsal glutinous ridge flower with the anthers in the process 
of lengthening out. b , flowers in the male stage before opening, c , flower fully mature 
with the pollen pushed out and the spreading lobes of the corolla, d, pollen grains. 
the base of the tube of the corolla, which is true, also, in 
many other Compos itce. 
Halsted* describes the phenomenon of the sensitiveness 
of the stamens as follows: 
la the thistle these filaments are slender, and are the parts in which 
motion is induced when the tip or other portion of the flower is touched. 
These filaments are colorless, and consist of cells from two to three times as 
long as broad, placed end to end and surrounding a central vascular bundle, 
consisting of six to ten very small closely coiled spiral vessels. In a cross- 
section, which is nearly triangular, the cells are seen to be thick-walled; 
those upon the hypothenuse of the right angle triangle — the inner side of 
the filament — being larger and thicker-walled than elsewhere. The exterior 
of this filament bears very many hairs, each always consisting of two nearly 
parallel cells extending side by side the whole length of the trichome. 
Usually one cell is longer than the other, and has the tip enlarged above the 
*Bull. Io. Agrl. Coll. Dept. Botany, Nov. 1886:29. 
