" The Weeds of South Africa." 73 



THE WEEDS OF SOUTH AFRICA." 



Notes on " Canada Thistle " 

 (" Cnicus Arvensis, " Hoffm.— Family " Conipositae "). 



By I. B. Pole Evans, M.A. D.Sc, F.L.S., Chief, Division of 

 Botany and Plant Pathology, and K. Lansdell, Division of 

 Botany. 



[NOTK. — It is intended to publish these notes, with coloured plate, in the series of 

 articles entitled "The Weeds of South Africa," issued by the Department as Local Series 

 Bulletins. — Acting Editor.] 



The " Canada Thistle," also termed " Thistles," " Cursed 

 Thistle," " Green Thistle," " Creeping- Thistle," " Prickly Thistle," 

 is of European origin. 



It is found in most countries in the world, including South 

 Africa, where at present it has only been reported from the borders of 

 Natal, the Transvaal, and from the Cathcart Division in the Cape 

 Province. 



Without doubt, it is one of the worst weed pests known to agri- 

 culture. Nearly every European country has wrestled with it, as 

 well as the United States, Canada, and New Zealand. 



The most noxious part of the " Canada Thistle " is the horizontal 

 creeping root stock. These are round and slender, and look like 

 tough, white whip cords lying 2 to 3 feet underground. They creep 

 in every direction, often to a length of a dozen feet or more, and send 

 up new plants at short intervals. Its rapid spread is due to these root 

 stocks, while each cut joint is able to produce a new set of plants. 



The plant is an erect perennial, growing to a height of 1 to 4 feet. 

 The stem is slender and free from prickles. The leaves at first appear 

 in a dense cluster, then alternately along the stem. They are from 3 

 to 5 inches long, very much cut, and armed with sharp spines along the 

 margin, vivid g'reen in colour on the surface, lighter in colour beneath. 

 The flowers are " thistle-like," and borne on long flower stalks, which 

 are smooth. The flowers are male and female, and are borne on 

 different plants. They are nearly always purple in colour, although 

 white flowers may sometimes be found. 



The " seed " is comparatively smooth, of a light brown colour, 

 slightly curved to one side. The apex is equipped with bristle-like 

 hairs called the "Pappus"; when these hairs fall off the apex is 

 cup-shaped, with a pointed tubercle in the centre. The body of the 

 seed tapers slightly to the base, which is round. The plant is pro- 

 pagated by seed, and the creeping root stock. The former are dis- 

 tributed by birds, threshing* machines, sheep, hay, agricultural seeds, 

 and manure. 



In America and New Zealand every means known has been adopted 

 to cope with the spread of this plant. The following extract on the 



