70 CIVIC BIOLOGY 



and study Avitli reference to means of dispersal. These collec- 

 tions will also be useful in identifying impurities in garden 

 and field seeds. 



Vitality of weed seeds. The great length of time weed 

 seeds remain alive in the soil furnishes another strong reason 

 for not permitting them to ripen. Dr. Beal has found that 

 seeds of pigweed, black mustard, shepherd's-purse, pepper- 

 grass, mayweed, evening primrose, smartweed,|)urslane, curled 

 dock, foxtail, and duckweed are able to germinate after being 

 buried for twenty-five years. How much longer they may 

 survive remains to be determined. 



Adaptability. Weeds, like other plants, attain their best 

 growth under favorable conditions of soil, but they are able to 

 produce seed under conditions of greatest hardship. A search 

 of clay banks, ash heaps, gravel or cinder walks will reveal 

 minute dwarf specimens of common weeds. Weeds also show 

 extreme tenacity of life in many other ways. Uproot them, 

 and if a rootlet remains in the soil, it will continue to grow 

 and mature seed as it lies on the ground. If trampled down, 

 branches that retain a shred of connection with the stem re- 

 main fresh and produce seed. Even when cut close to the 

 ground, new branches quickly appear. An outdoor lesson 

 devoted to this topic will prove valuable in many connections. 



Classes of weeds. Success in fighting weeds must depend 

 upon knowledge of each species. The methods used will de- 

 pend in a general way upon wliether the particular weed is : 



An annual Most garden weeds — duckweed, purslane, 

 lamVs-quarters, pigweed, ragweed, pigeon grass — complete 

 their life cycle in a single season. To completely eradicate 

 these from the soil it is only necessary to prevent any from 

 maturing seed until seeds already in the ground have germi- 

 nated. Barnyard manure or loam may reseed a clean garden 

 plot, if not applied with great care. If manure is spread upon 

 the top after plowing, all weed seeds in it are more likely to 



