Returning to the witness stand, a scientist on Wednesday defended her opinion that Monsanto’s popular weed killer Roundup causes cancer.  UCLA epidemiologist Dr. Beate Ritz was invited back to court after she and other scientists spent a week testifying in March about whether glyphosate, the active ingredient in the world’s most popular weed killer, causes Non-Hodgkin’s Lymphoma in humans.  Ritz’ testimony could advance more than 300 lawsuits filed against Monsanto and consolidated in a multidistrict case in San Francisco.  The suits were filed by farmers and landscapers who say exposure to Roundup’s active ingredient caused them to develop cancer of the lymph nodes...U.S. District Judge Vince Chhabria must decide whether there is adequate scientific evidence to support the plaintiffs’ claims against Monsanto...Ritz was responding to criticism from Monsanto and Chhabria, who last month dismissed some of her opinions as “junk science” because she failed to account for exposure to carcinogenic pesticides in her analysis...