FIG. 502. Mixture of weed seeds commonly found in low grade Alsike Clover seed: a, Alsike Clover; b, White Clover; c, Red Clover; d, Yellow Trefoil; e, Canada Thistle; f, Dock; g, Sorrel; h, Buckhorn; i, Rat Tail Plantain; k, Lamb's Quarters; 1, Shepherd's Purse; m, Mayweed; n, Scentless Camomile; o, White Campion; p, Night-flowering Catchfly; q, Ox-eye Daisy; r, Small-fruited False Flax; s, Cinquefoil; t, two kinds of Peppergrass; u, Catnip; v, Timothy; x, Chickweed; y, Canada Blue Grass; z, Clover Dodder; 1, Mouse Ear; 2, Knotgrass; 3, Tumbling Amaranth; 4, Rough Amaranth; 5, Heal-all; 6, Lady's Thumb (enlarged). (Hillman, U. S. Dept. Agr.) FIG. 503. Seeds of Poas with impurities. 1, Kentucky Blue Grass rubbed and unrubbed; 2, Wood Meadow Grass (Poa nemoralis); 3, Ergot, a fungus; 4, Texas Blue Grass (P. arachnifera); 5, Canadian Blue Grass (Poa compressa); 6, Rough Stalked Meadow Grass (Poa trivialis); 7, Silky Bent Grass (Apera spica-venti); 8, Wood Hair Grass (Deschampsia flexuosa); 9, spine of Canada Thistle; 10, Canada Thistle; 11, caryopsis of Stink Grass (Eragrostis major). (U. S. Dept. Agr.) weeds have not prominent dissemination mechanisms but are transferred with agricultural seeds, as carrot, buckhorn, dodder, sheep sorrel, yellow trefoil, quack grass and pigeon grass. The separation of impure seed from agricultural seed necessitates the recognition by their morphological characters of obnoxious seeds. An economic consideration of dissemination of weed seeds and their control involves (1) familiarity with the morphology of the fruit and seed; (2) the limitation of the habitat in which the weed is able to thrive; (3) recognition of the avenues and agents of distribution. |