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 proportions. 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 efi'ectiveness is destroyed by the adulter- 

 ants which are so readily identified by the compound microscope. 



POWDERED WTIITE 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 papillas 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 cells 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 papillae 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 cefls (Fig. 10), which 

 in the mature form have thick white walls and scarcely any 



