THE MORPHOLOGY OF WEEDS 1 15 



The following are illustrations of this type: Awned 

 brome grass, black medick, bracted plantain, brome 

 grass, bur clover, buffalo bur, cheeses, chess, cockle, 

 cocklebur, common sunflower, cowherb, dooryard 

 knotweed, dog fennel, fetid marigold, Fuller's teasel, 

 goosefoot, great ragweed, green foxtail, hairy brome 

 grass, hedge mustard, horseweed, Jimson weed, marsh 

 elder, Mexican fireweed, mustard, pepper grass, pigeon 

 grass, pigweed, prairie bitterweed, prickly lettuce, purs- 

 lane, Rocky Mountain bee plant, Russian thistle, small 

 ragweed, smartweed, squirrel-tail grass, tumbling 

 mustard, velvet weed, vetch, wild barley, wild oats. 



The seeds of winter annuals ger- ^j^^ .^^ 

 minate in the fall, the plants live 

 through the winter and in the spring 

 produce flowers and seeds; shepherd's 

 purse, speedwell, winter rye and win- 

 ter wheat are types of this class. Such 

 plants are more common southward 

 than in the North. Plants which in 

 the North are annuals may become 

 winter annuals in the South. 



The seeds of biennials germinate in 

 the spring, producing a growth of short 

 stems and leaves the first season, but Fig. 37. Root hair, 

 no flowers; in the second season the (Leavitt.) 

 stem elongates, produces flowers and seeds. The follow- 

 ing weeds are biennial: Bull thistle, burdock, mullein, 

 Nelson's thistle, parsnip, and sweet clover. Other repre- 

 sentatives: Cultivated beet, cabbage, carrot, and parsnip. 



Perennial plants produce roots which continue to live 

 year after year. All of our trees and shrubs and the fol- 

 lowing weeds are perennial : Blue lettuce, buckhorn or 

 ribgrass, Canada thistle, common nettle, common plan- 

 tain, cowbane, curled and smooth dock, dandelion, dog- 

 bane, gaura, germander, horse nettle, ironweed, milk- 



