account of the great number of superimposed cells. The parenchyma of the receptacle occurs in fragments which have strongly marked porous walls. OPEN INSECT FLOWERS Many of the structures of open insect flowers (Plate 119) are similar to those found in the closed flower. There is practically no difference in the edge of the scale (Fig. 1); or the fibre of the scale (Fig. 2); or the T-shaped hairs (Fig. 3); or the upper epidermis of the ray flower (Fig. 4); or the under epidermis of the ray flower (Fig. 5); or the cross-section of the ray petal (Fig. 6); or the lobe of the disk petal (Fig. 7); or the filament tissue (Fig. 8); or the lobe of the stamen (Fig. 9); or the papillæ of the stigma (Fig. 12); or the parenchyma of the receptacle (Fig. 15). The difference in structure is found, first, in the involucre scales, which are more fibrous than the scales of the closed flowers; secondly, in the pollen (Fig. 11), which is less abundant than in the closed flower; it is also lighter in color and usually shows the wall perforated by three pores; thirdly, the outer layers of the achene consist of thick, porous-walled stone cells (Fig. 13), which occur singly or in groups; fourthly, 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. [Illustration: PLATE 119 POWDERED OPEN INSECT FLOWER (_Chrysanthemum cinerariifolium_, [Trev.] Vis.) 1. Edge of involucre scale. 2. Fibres of involucre scale. 3. Hairs. 4. Upper epidermis of ray flower. 5. Under epidermis of ray flower. 6. Cross-section of ray petal. 7. Lobe of disk flower. 8. Filament tissue. 9. Lobe of stamen. 10. Calyx tissue, 11. Pollen. 12. Papillæ of the stigma. 13. Stone cells from the achene and cross-section of achene. 14. Secretion cavity with surrounding cells. 15. Parenchyma of the receptacle.] Insect flowers are the most valuable vegetable insecticide known; yet much of its effectiveness is destroyed by the adulterants which are so readily identified by the compound microscope. POWDERED WHITE DAISIES