284 HISTOLOGY OF MEDICINAL PLANTS 
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. 
When studying flowers there should be considered the number 
and structure of pollen grains; the nature of the papille 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. 
