What is the name for the type of venation where there is a single main vein that runs through the center of the leaf? 

Venation refers to the arrangement of veins in a leaf that facilitates the transport of water, nutrients, and sugars throughout the plant. Simple venation is the term used to describe the type of venation where there is a single main vein that runs through the center of the leaf. This type of venation is common in simple leaves, such as those of dicot plants. In contrast, pinnate venation is characterized by multiple veins branching off from a central midrib, palmately compound venation features several veins branching from a single point at the base of the leaf, and reticulate venation is a web-like network of veins found in broad, flat leaves.