Powdered Insect Flowers

The microscopic examination of insect powder is difficult for the reason that there are so many elements to be constantly kept in mind. The parts of the flower which contribute characteristic cells are the stem, involucre, ray flowers, disk flowers, and the receptacle. In each of these parts there are many different types of cells.

There are practically two types of flowers found in insect powder of commerce: first, closed or immature flowers, and secondly, open or mature flowers. As explained above, the half-open flowers consist largely of the two above-named varieties. Let us first consider the structure of the closed insect flowers as illustrated in Plate 118.

The involucre has many characteristic cells. The more prominent ones seen in the powder are the edge of the scale with the attached hair (Fig. 1). These hairs (Fig. 3) are T-shaped. The terminal cell is expanded laterally, and it terminates in two points. Connecting the terminal cell with the epidermis are two or three cells which are slightly longer than broad. In the powder the terminal cell is usually attached to fragments only of the supporting cells. Fibres of the bracts have thick, wavy, porous walls, and they have a tendency to occur in masses. The upper epidermis (Fig. 4) of the ray-flower petal is prominently papillate. The under epidermis consists of wavy cells without papillae. Another view of the papillae is shown in Fig. 6. The parenchyma of the ray flowers (Fig. 7) contain cubical crystals. The lobe' of the disk-flower petal (Fig. 8) is papillate at the end, the terminal cells have thick outer and thin inner walls. The filament tissue (Fig. 9) is composed of nearly square cells. The calyx tissue (Fig. 10) is made up of thin-walled cells with slightly papillate margins. The lobe of the stamen (Fig. 11) consists of nearly uniform epidermal cells which are in contact throughout their long diameter, while the hypodermal cells are thin-walled and angled. The pollen grains (Fig. 12) are dark yellowish green, thin, and the wall does not appear perforated by pores. The papillae of the stigma (Fig. 13) are clustered, club-shaped, and nearly white in color. They are usually found detached in the powder. All parts of the pistil contain secreting cells, but the most conspicuous secreting cavities (Fig. 14) are those of the ovary. These cavities appear brownish in color and are surrounded by small cells which appear indistinct on account of the great number of superimposed cells. The parenchyma of the receptacle occurs in fragments which have strongly marked porous walls.

Plate 118. Powdered Closed Insect Flower.

{Chrysanthemum cinerariifolium, [Trev.] Vis.)

I. Edge of scale. 2. Fibre of scale. 3. Hairs. 4. Upper epidermis of ray flower. 5. Under epidermis of ray flower. 6. Cross-section of ray petal. 7. Parenchyma of ray flowers with crystals. 8. Lobe of disk petal. 9. Filament tissue. 10. Calyx tissue. II. Lobe of stamen. 12. Pollen. 13. Papillae of stigma. 14. Secretion cavity with surrounding cells. 15. Parenchyma of the receptacle.

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 papillae 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.

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. II. Pollen. 12. Papillae 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

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 somewhat similar to those found in insect flowers. These structures are the papillae 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 papillae 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 cavity. These cells swell perceptibly when placed in water. The other striking feature of the achene is the bright red resin masses which occur free in the field. Even a small trace of daisies in insect powder can be identified.

Plate 120. Powdered White Daisies (Chrysanthemum leucanthemum, L.).

1 and 2. Scale tissue. 3, 5 and 13. Papillae of petals. 4. Scale tissue. 6. Lobe of ray petal. 7. Filament tissue. 8. Pollen. 9. Papillae of stigma. 10. Palisade cells of achene. II. Resin masses. 12. Parenchyma of receptacle. 14. Lobe of dish petal.

When studying flowers there should be considered the number and structure of pollen grains; the nature of the papillae of the stigma and the petals; the nature of the hairs of the corolla and calyx, when present. In the composite flowers we should also consider the structure of the involucre scales, and, when present, the structure of the receptacle scales, as in the case of anthemus, and of the pappus hairs, as in the flowers of arnica, boneset, grindelia, and aromatic goldenrod.