Bromus catharticus (rescuegrass)

Senecio viscosus (sticky ragwort)
Senecio vulgaris
Senegal
Senna marilandica (Maryland senna)
Senna occidentalis (septicweed)
Sequoiadendron giganteum
Serbia
Sericocarpus asteroides (toothed whitetop aster)
Sericocarpus linifolius (narrowleaf whitetop aster)
Serratula tinctoria
Seseli montanum
Sesleria nitida
Sesuvium maritimum (slender seapurslane)
Setaria parviflora (marsh bristlegrass)
Setaria pumila
Setaria pumila ssp. pumila (yellow foxtail)
Setaria verticillata (hooked bristlegrass)
Setaria viridis var. viridis (green bristlegrass)
Seychelles
Sherardia arvensis
Sherardia arvensis (blue fieldmadder)
Shinnersoseris rostrata (beaked skeletonweed)
Sibara virginica (Virginia winged rockcress)
Sibbaldia procumbens (creeping sibbaldia)
Sida elliottii (Elliott's fanpetals)
Sida hermaphrodita (Virginia fanpetals)
Sida spinosa (prickly fanpetals)
Sierra Leone
Silene antirrhina (sleepy silene)

Image of Bromus catharticus (rescuegrass) 1
USDA-NRCS PLANTS Database / Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. Vol. 1: 280.