- Rosaceae, [138]
- Rose family, [138]
- Rough cinquefoil, [139]
- Rough pigweed, [127]
- Rudbeckia, [169]
- Rugel's broad-leaved plantain, [159]
- Rumex, [121], [122], [123]
- Running mallow, [146]
- Rush family, [118]
- Rush, slender, [118]
- Rush, spike, [118]
- Russian thistle, [126]
- Rutabaga, [133]
- Rye, [116]
- Salsify, [170]
- Salsola, [126]
- Sand-bur, [112]
- Sand plantain, [158]
- Sand rocket, [135]
- Sandwort, [129]
- Saponaria, [129]
- Scarlet clover, [141]
- Scleria, [128]
- Scrophulaceae, [156]
- Secale, [116]
- Sedge family, [118]
- Sedum, [138]
- Self-heal, [155]
- Setaria glauca, viridis, [117]
- Sheep sorrel, [122]
- Shepherd's purse, [134]
- Sherardia, [159]
- Shore knot-weed, [120]
- Sida, [146]
- Silene, [129], [130]
- Silvery cinquefoil, [138]
- Sisymbrium, [137]
- Sleepy catchfly, [129]
- Slender pigweed, [127]
- Slender nettle, [119]
- Slender rush, [118]
- Small alyssum, [132]
- Smaller burdock, [162]
- Small-flowered crane's bill, [142]
- Small-flowered crowfoot, [131]
- Small-fruited false flax, [134]
- Smut-weed, [120]
- Solanaceae, [155]
- Solanum, [155], [156]
- Sonchus, [169]
- Sorrel, [121]
- Sour dock, [121]
- Sow-thistle, [169]
- Spanish dodder, [151]
- Spear grass, [116]
- Speedwell, [157]
- Spergula, [130]
Spike´let, a small or secondary spike, as found in grasses.
- Spotted spurge, [144]
- Spring clotbur, [171]
- Spring sow-thistle, [169]
- Spurge family, [143]
- Spurry, [130]
- Squirrel-tail grass, [115]
- St. John's-wort family, [147]
- Star thistle, [163]
- Stellaria, [131]
Ster´ile, not fertile.
Stri´ate, striped with parallel ridges and grooves.
- Swallow-wort, [149]
- Swamp begger-ticks, [163]
- Sweet clover, [140]
- Sweet everlasting, [166]
- Syntherisma, [113]
- Tall buttercup, [131]
- Tall mustard, [137]
- Taraxacum, [170]
- Teasel family, [160]
- Thistle, [164]
- Thistle, Russian, [126]
- Thorn apple, [155]
- Three-seeded mercury, [143]
- Thyme-leaved sandwort, [129]
- Thyme-leaved speedwell, [157]
- Thyme-leaved spurge, [144]
- Toad-flax, [156]
- Tragopogon, [170]
- Treacle mustard, [135]
- Trefoil, [139]
- Trifolium, [141], [142]
Trun´cate, terminating abruptly, as though cut off or flattened at the end. Compare Premorse and Succise.
Tu´ber-cle, a swollen part or a root due to bacteria. Usually applies to such as possess the power to fix nitrogen; a little tuber.