The Midrib provides the principal Vascular Connection to the Stem. Their Vascular Organization  varies from Simple (one vascular bundle) to Complex.

Ohi'a lehua Midribs viewed with Bright-Field and Polarizing Optics

The Midrib in ti (Monocot) has an Assemblage of Many Veins

Midrib of Banana (Musa)
Note the numerous Fibrovascular Bundles & Bundles of pure Fibers which provide support for this incredibly large leaf.

Top of Musa Midrib
Note the abundance of Fibrovascular Bundles and the absence of Fiber Bundles

  
Lower part of Musa Midrib
Notice the many Fiber Bundles on the Lower part of the Midrib Note the Lignification (Blue Areas)

The Midrib also provides Structural Support for the Leaf. This is due to the presence of Collenchyma & Sclerenchyma & Xylem.