ROOTS AND ROOT-STOCKS OF WEEDS 



645 



many usually enlarged and fleshy roots. Sweet clover {Melilotus 

 alba), Fig. 505, offers a good example of this type. Third, tuberous 

 or those that develop an enlarged portion at the end of la some- 

 what smaller one. Fourth, fibrous in which a mass of small roots 

 develop usually just below the stem as in most of the grasses and in 

 the plantains. 



Besides the forms already mentioned we find various types of 

 aerial roots, or those that develop on the plant above ground. The 

 aerial roots of the ivy become finger-like and cling to objects to 

 assist the plant in climbing ; those of the mistletoe and the dodder 

 become parasitic by pushing their aerial roots into the tissues of 

 the host plant and drawing nourishment from it. 



In performing the four functions mentioned above the roots of a 

 single plant may occupy considerable area. Dr. Pammel has pre- 

 pared the following table showing the depth and spread of the 

 root system of a number of our noxious weeds. 



Weed 



Buckhorn 



Plantain 



Wild hemp 



Evening primrose 



Beggar ticks 



Dog fennel 



Nigger head 



Goldenrod 



White vervain 



Canadian lettuce 



Field thistle 



Burdock 



Black nightshade 



Pennsylvania smartweed 



Lady's thumb 



Yellow oxalis 



Prickly lettuce 



Cocklebur 



Greater ragweed 



Rough pigweed 



Horseweed 



Tumbleweed 



Small ragweed 



Spanish dagger 



The amount of reserve food stored within weed roots depends 

 to a considerable extent upon the length of time they continue to 

 live. 



