220 
IOWA ACADEMY OF SCIENCES. 
end of its mate. A common hyaline coating covers both the hair cells. 
These cells do not end at the base of the outgrowth, but one passes up the 
filaments and the other in the opposite direction, meeting other epidermal 
cells that are not modified into hairs. The origin of these outgrowths is 
easy to find. All stages of their development may be seen in any young 
filament. At first only a slight elevation is observed at the point where two- 
Fig- . 13. Plumose bristles, portion of pappus, a single thistle. 
epidermal cells join ends. A little later these ends push out side by side,, 
closely appressed, thus making a papilla. Soon after this the lateral por- 
tion of each cells equals that which is epidermal. Nearly all the granular 
contents of both cells are now in the projecting part and the nucleus is 
prominent. When fully developed the hairs are 20 to 25 u. broad and 1,000 
u. long. They are, therefore, the laterally extended tips of adjoining epi- 
dermal cells and become triggers, which, when touched by the insect’s pro- 
boscis, set the filament in motion and cause the drawing down of the tube 
of anthers. The style, it has been stated, occupies the space within the 
tube and has its exterior covered with short stiff hairs which point upward. 
A brush of long upturned hairs is situated at the base of that portion of 
the style included within the ring of anthers. The anthers dehisce upon 
the inner side and the pollen comes in contact with the “spiny” surface of.: 
