 The domestication of lettuce over the centuries has resulted in several changes through selective breeding : delayed bolting , larger seeds , larger leaves and heads , better taste and texture , a lower latex content , and different leaf shapes and colors . Work in these areas continues through the present day . Scientific research into the genetic modification of lettuce is ongoing , with over 85 field trials taking place between 1992 and 2005 in the European Union and United States to test modifications allowing greater herbicide tolerance , greater resistance to insects and fungi and slower bolting patterns . However , genetically modified lettuce is not currently used in commercial agriculture . 
