The histological structure of flowers is readily seen in the powder; therefore, in studying flowers, it is not necessary to section the various parts. Each part of the flower should be isolated and powdered separately and each separated part studied. In each case the powders will contain surface, cross-, and radial sections of the parts powdered. While studying flowers, special attention should be given to the pollen grains, to the papillæ of the petals, to the papillæ of the stigma, and, in certain flowers, to the style tissue. In the composite flowers special attention should also be given to the involucre scales, to the scales of receptacle, and, when present, to the pappus. In addition, attention must be given to secretion cavities, as in cloves. POLLEN GRAINS Pollen grains are one of the most characteristic elements found in powdered flowers, because they are so small that they are not broken up when the drug is milled. The two principal groups of pollen grains are, first, those with non-spiny walls (Plate 113); and, secondly, those with spiny walls (Plate 114), as shown in the two charts. In lavender flowers the pollen grains have six constrictions of the outer wall. This wall is slightly striated and the cell contents are granular. In clover flowers the pollen grains are mostly rounded in outline, the wall is uniformly thickened, and cell contents are coarsely granular. In belladonna flowers the pollen grains terminate in three blunt points. In Spanish saffron the pollen grains are spherical and the cell contents are granular. [Illustration: PLATE 113 SMOOTH-WALLED POLLEN GRAINS 1. Cloves (_Eugenia caryophyllata_, Thunb.). 2. Santonica (_Artemisia pauciflora_, Weber). 3. Elder (_Sambucus canadensis_, L.). 4. Century minor (_Erythræa centaurium_ [L.], Pers.). 5. Pichi (_Fabiana imbricata_, R. and P.). 6. Cyani. 7. Lavender (_Lavandula officinalis_, Chaix.). 8. Clover (_Trifolium pratense_, L.). 9. Belladonna (_Atropa belladonna_, L.). 10. Spanish saffron (_Crocus sativus_, L.).] [Illustration: PLATE 114 SPINY WALLED POLLEN GRAINS 1. Anthemis (_Anthemis nobilis_, L.). 2. Arnica (_Arnica montana_, L.). 3. Calendula (_Calendula officinalis_, L.). 4. Cassia flowers. 5. American saffron (_Carthamus tinctorius_, L.). 6. Blue malva flowers (_Malva sylvestris_, L.).] The non-spiny-walled pollen grains differ not only in microscopic appearance, but also in size. Clove pollen grains are the smallest, while Spanish saffron pollen grains are the largest. NON-SPINY-WALLED POLLEN GRAINS In cloves the pollen grains show a six-sided, angled cavity and an outer wall which terminates in three slightly pointed, narrowly notched portions, separated by nearly straight walls.