Hyphae that grow horizontally along the surface of a food source are called stolons.

Here's why:

* Stolons are specialized hyphae that grow horizontally and act as runners, spreading out across the surface of the food source. They help to spread the fungus and find new areas for nutrient absorption.

* Rhizoids are root-like hyphae that anchor the fungus to the substrate.

* Aerial hyphae are hyphae that grow above the surface of the substrate and are involved in reproduction.

So, while rhizoids and aerial hyphae play different roles, stolons are the specific type of hyphae that grow horizontally to spread across the surface of a food source.