Five stories that have North Texas talking: a high-profile immigration debate; tougher rules on fertilizer following the West explosion?; conjoined twins will soon go home; and more:

• The University of North Texas has acquired hundreds of thousands of images amassed by four generations of photographers in one Fort Worth family. The Fort Worth Star-Telegram reports Byrd Williams IV, his father, grandfather and great-grandfather all lived in Fort Worth and used cameras to earn a living, document history and create art. The collection dates to the late 1800s. The university acquired it in recent months for an undisclosed amount. Morgan Gieringer, head of archives and rare books at UNT in Denton, says about 80 percent of the collection documents Fort Worth, including portraits and photos of events and architecture. The collection also includes documents other parts of Texas, including photos by Byrd Williams II of soldiers fighting with Mexican revolutionary Pancho Villa near El Paso in 1915. [Associated Press]

• Tonight, a prominent immigration reform advocate faces a prominent Republican hardliner on immigration reform. The high-profile debate pits San Antonio Mayor Julian Castro against state Sen. Dan Patrick, a Houston Republican competing against Lt. Gov. David Dewhurst in a May 27 GOP primary. Castro advocates granting legal residency to young people brought by their parents into the United States without papers. Castro has been critical of Patrick, a tea party-backed radio talk show host who has decried what he calls the “invasion” of immigrants coming across the Texas border. Patrick was dogged shortly before the March 4 first primary by allegations, which he denied, that he knowingly hired immigrants in the country illegally at bars he owned in the 1980s. The San Antonio Express-News has more. The Texas Tribune will livestream the 6 p.m. event in Houston. The Tribune has a preview. [Associated Press]

• How low did it go overnight? At 7 a.m., D/FW International Airport recorded 37 degrees – not below freezing and not quite close to setting a record. The last time we recorded freezing temperatures at D/FW this late in April was back in 1997 – 32 degrees on April 13. At 7 a.m., Denton was at 30 degrees. Graham was at 26. Waco was at 31 degrees, which ties Waco’s record low for April 15, which had been set in 1983. Waco has had 70 freeze days this season – the greatest number on record for that city, the National Weather Service says. While North Texas didn’t set records, we were 15 to 20 degrees below normal this morning.

• Investigators told a Texas House committee Monday that they’re still not sure what sparked the fire that sparked the West plant explosion that killed 15 people. But  Chris Connealy, the state fire marshal, told legislators that the law needs to changed to prevent something like the West explosion from happening again: Either require fire sprinklers or store ammonium nitrate in fire-resistant storage bins. Rep. Joe Pickett, chairman of the House Homeland Security and Public Safety Committee, said Connealy’s proposal is expected to be included in a draft version of a bill that would be rolled out this summer for debate, The Dallas Morning News reports. The Legislature convenes in January. NPR’s Wade Goodwyn recently visited West as the town approaches the one-year anniversary of the plant explosion. Pickett told NPR that a sprinkler system requirement could have a chance of passing the Texas Legislature, but he’s not sure.

• The conditions of conjoined twins separated last summer have steadily improved, and officials say they’ll be released this week from Medical City Children’s Hospital. Officials announced Monday that Owen and Emmett Ezell are expected to be discharged Wednesday. They were born in July, joined at the abdomen. The boys are no longer being fed through an IV but continue to be fed through tubes in their abdomens. And instead of being hooked to breathing machines, they now need only the assistance of a trachea tube. The boys will move from the hospital to a rehab center. The babies shared a liver and bowels, and they had a birth defect that left their intestines outside of their bodies and covered by a thin layer of tissue. [Associated Press]