HOW WEEDS ARE SPREAD 19 



period will flower and seed. Examples of biennial 

 weeds are : Marsh Thistle, Burdock, Hemlock, and 

 Foxglove. 



Perennials are those plants which live and continue 

 to produce flowers and seeds for a number of years in 

 succession ; they grow from seed, and may be propa- 

 gated and spread by means of their rootstock and 

 other organs. Examples are : Couch Grass, Coltsfoot, 

 Ox-eye Daisy, Creeping Crowfoot, Creeping Thistle, 

 Knapweed, Yarrow, Bindweed, and Stinging Nettles. 



These three classes of weeds may be spread or dis- 

 tributed in a great variety of ways, the chief of which 

 are given below. 



(a) Weeds are Spread by Natural Seeding and by the 

 Wind. In the ordinary course of events weeds pro- 

 duce seeds at their normal seasons, such seeds being 

 naturally distributed over a narrow or wide area ac- 

 cording to certain botanical characters of the plant. 

 Many seed-vessels are so constructed that on ripening 

 they split and throw out their seed with considerable 

 force, projecting it to a distance from the parent plant. 



Other seeds bear flight organs or are " winged," 

 and are carried through the air for long distances by 

 the wind (e.g. seeds of Thistles, Groundsel, Dandelion, 

 Yellow Rattle, and Dock). Some seeds, such as those 

 of Broom-rape and Poppy, are very small and light, and 

 are readily scattered by the agency of the wind. (See 

 Plates I., II., and III., Weed Seeds.) 



(b) Weed Seeds are Distributed by Natural Agencies. In 

 addition to the transport of seeds by the wind may be 

 mentioned the conveyance by rivers, streams, heavy 

 rains, and floods which often wash seeds away from 

 the parent plants and carry them long distances. 



Some seeds, or fruits, bear numerous little hooks 

 by means of which they become attached to passing 



