282 HISTOLOGY OF MEDICINAL PLANTS 
the secretion cavity is broader and darker in color (Fig. 14). 
These differences enable one at once to distinguish between 
the closed and open insect flowers. Now, since the half-closed 
flowers consist almost wholly of a mixture of equal parts of 
closed and open flowers, it follows that the elements of these 
two flowers will be mixed in about equal proportiens. Thus 
we are able to distinguish microscopically the three commercial 
varieties of insect powder—namely, closed insect flowers, open 
. insect flowers, and half-open insect flowers. 
Insect flowers are the most valuable vegetable insecticide 
known; yet much of its effectiveness is destroyed by the adulter- 
ants which are so readily identified by the compound microscope. 
POWDERED WHITE DAISIES 
A common adulterant found in open insect flowers is the 
flower-heads of European daisy (C. leucanthemum). Examination 
of powdered flowers exported from Europe shows that the entire 
flower-head is ground and mixed with the insect flowers. In 
the cheaper varieties of open flowers, only the tubular flowers 
are added after they have been separated from the heads by 
crushing and sifting. These tubular flowers so closely resemble 
the tubular flowers of the true insect flowers that it is practically 
impossible to distinguish between them macroscopically. The 
quickest and surest way to identify them is to reduce a portion 
of the flowers to a fine powder and examine it microscopically. 
Certain structures of the white daisies (Plate 120) are some- 
what similar to those found in insect flowers. ‘These structures 
are the papilla of the ray petal (Figs. 3, 5, and 13), the lobe of 
the disk petal (Fig. 14), and the lobe of the stamen and the 
pollen (Fig. 8). 
The differences are as follows: The under epidermis of the 
ray flowers is composed of wavy celis which are more elongated 
than the ray flowers of the under epidermis of the ray petal of 
insect flower. The filament tissue is made up of slightly beaded 
cells instead of smooth-walled cells. The papilla of the stigma 
are smaller than the papilla of insect flowers. The most striking 
difference is found in the structure of the achene. The epidermal 
tissue of the achene is composed of palisade cells (Fig. 10), which 
in the mature form have thick white walls and scarcely any 
