WEEDS— PECULIARITIES AND DISTRIBUTION 419 



nodes ; roots then arise at each node, so that each rooted 

 segment is capable of forming a new plant, which eventually 

 may become detached from the parent plant and lead a separate 

 existence. This is well seen in the Creeping Buttercup, the 

 Wild Strawberry and the Silverweed. In other cases, the 

 runners are hidden away beneath the surface of the soil, 

 forming rhizomes or underground stems which push up aerial 

 shoots at frequent intervals. One of the worst of such pests 

 is the Couch or Twitch {Triticum rcpens), which sends out long- 

 underground stems bearing scale leaves from whose axils aerial 

 branches spring up. The whole plant is most tenacious of life, 

 every bit of the rhizome having the power when broken off 

 of rooting itself and forming a new and independent plant. 

 It seems as if nothing will kill it ; the only means of eradicating 

 it is to cultivate and remove most carefully ever}' bit that is pos- 

 sible of the plant. The Enchanter's Nightshade, which is very 

 common in woods and is also locally a serious pest in gardens, 

 is chiefly reproduced in the same way, its long thin rhizomes 

 penetrating the ground in every direction. 



Another plant belonging to the Grass family is known as 

 "Knotty Couch" or "Onion Couch" 1 because of the peculiar 

 way in which it spreads. It is closely related to the Tall 

 Oat-Grass and is in fact a sub-species of this genus. The base 

 of the stem or rootstock swells up into thick knobs of which 

 there may be several in a string one above the other. Each 

 one of these knobs is capable of giving rise to a new plant 

 if it be detached from its fellows. Consequently the Knotty 

 Couch is as great a pest as is the true Couch. It has been 

 supposed that the Onion Couch and the Tall Oat-Grass are 

 really one and the same plant and that the knotted form is 

 merely the result of local conditions, constitution of the soil 

 and the amount of available soil moisture and rain ; however, 

 experiments have been made by growing seed from the two 

 forms under various conditions of soil, moisture and light and 

 it has been found that both types always come true to seed, 

 so that it must be admitted that the Knotted form is a distinct 

 variety which breeds true under different conditions and does 

 not revert to the unknotted type. 2 



1 Arrhenatherum avenaceum var. tuberosum. 



2 I am indebted to Miss Underwood, Rothamsted Experimental Station, for 

 information relating to these experiments. 



