Pollen of hoary alyssum (Berteroa incana), a perennial mustard family member with small white flowers and hairy, grayish-green leaves and stems. The signs of toxicity to horses include depression and stocking up or swelling of the lower legs, which may occur 12 to 24 hours after feeding on hoary alyssum. Enhanced SEM. Image width: 81 micrometers. Magnification: 1234x if the image is printed 10 cm wide.