DISTRIBUTION OF WEEDS 



39 



existence. Sooner or later the connecting stems may perish , but 

 they often persist for a long period, so that a single plant may 

 cover a considerable area of ground as the same process of 

 reproduction goes on repeatedly. The conditions of the habitat 

 largely determine the spread of these weeds. Under some 

 circumstances the runners remain short, and the daughter plants 

 are grouped closely round the parent, but in others the runners 

 are lengthy and carry the plantlets well away, thus increasing 

 the area of spread of the weed. 



(c) By Other Types of Vegetative Reproduction. Some 

 weeds have developed very specialised types of reproduction 



FIG. 22. MOUSE-EAR HAWKWEED (Hieracium pilosella). 

 A. Runners (above ground). 



which, though less common than the two preceding classes, 

 are quite effective in aiding the distribution of the species. 



The reproduction of the wild onion (Allium vineale] by 

 means of seeds and underground bulbs has been described, 

 but it has still a third method, as the flowering heads always 

 carry a number of small bulbils at the base of the flower stalks. 

 It often happens, indeed, that the flowers are entirely absent, 

 so that the " flower head " consists of a main stalk bearing 

 aloft a cluster of small bulbs. These ripen and fall to the 

 ground, where they start into growth and give rise to new 

 onion plants. It is largely because of this triple method of 

 reproduction that the wild onion is so difficult to exterminate. 



Onion or knotty couch (Arrhenatherum avenaceum, var. 



