Hello!

I was hoping to get some help answering a reference question for a patron that has stumped me. They are looking for a short story that was most likely published in the 1950s or 1960s. It is science fiction and is about a planet that humans land on, where the planet is controlled by a plant named "the core." She said it is about 5 pages long. It was most likely published in an anthology or magazine. 

At first I thought it might be "Vaster than Empires and More Slow" but that doesn't refer to the plant entity by name and was published in the 1970s. 

Many thanks in advance!