596 WEEDS : SPECIAL 



no broad wings running down from the leaves; flower heads 

 oval with light purple flowers. 



The fruits, sometimes named " seeds," contain a single seed, 

 and are surmounted by a number of silky hairs (thistle-down), 

 as at 3, Fig. 148. It appears to grow on all soils, but is most 

 difficult to eradicate from those of very loose nature. 



Sow - thistle. Milk - thistle {Sonchus oleraceus L.). — An 

 annual with tap root and erect branched stem bearing pinnatifid 

 toothed clasping leaves. The flower heads have yellow flowers 

 and smooth involucre. Another very similar species is Sonchus 

 asper Hofi"m. 



Com Sow-thistle (Sonchus arvensis L.). — A perennial with 

 creeping underground stem. The shoots above ground grow 

 3 to 4 feet high. The flower heads are yellow, but larger than 

 the preceding species, and their involucres are hairy. 



These three thistles are common on arable land and waste 

 ground, and like all others are rapidly spread by means of their 

 feathery-tipped fruits, which are specially adapted to be blown 

 about by the wind. Two of them are perennials, and are also 

 propagated by means of their underground stems. 



Seed production should be prevented. If cutting off the stems 

 is adopted for this purpose, it is essential that it should be done 

 before flowering if possible, as when left till later the shoots are 

 often able to ripen their seeds when severed from the root. 



It is very important to bear in mind that the remedies em- 

 ployed to prevent the seeding of thistles should be applied to 

 them not only on cultivated land, but on all waste land as far as 

 possible, as it is from the latter source that most of the continu- 

 ous supply of young plants infesting good ground is derived. 

 Hand-pulling or cutting below the hypocotyl is an efficient remedy 

 in the case of the annual species. In order to exterminate the 

 perennials absolutely, the complete underground stem must be 

 destroyed. They are, however, checked by hand-pulling, but as 

 they cannot be completely removed in this manner, the pieces 



