46 



BULLETIN 791, V. S. DEPARTMENT OF AGRICULTURE. 



tansy mustard, for example, have averaged more tlian 70 per cent, 

 considerably exceeding the highest average seed-test records of per- 

 ennial species. Generally, the seed of the more aggressive annuals 

 are plump and well filled with reserve food. Another distinct ad- 

 vantage observed in the case of the seed of the ruderal weed species 



Goosefoot 01" lamb's-quar 

 i€%enopoMum cWbum). 



ters 



Slender-leaved coUoiiiia 

 (Coltomlo, Ihieariii). 



Fig. 17. — Plants characteristic of tlie first -or early weed stage. 



is their ability to germinate under conditions of temperature ad- 

 verse to most plants. Seed of Douglas knotweed, tansy mustard, and 

 tarweed, for instance, have been observed to sprout when the maxi- 

 mum diurnal temperature has not exceeded 50° Fahrenheit, the 



