What is parallel and reticulate venation?

 Parallel Venation -

Parallel venation occurs when veins (or nerves) are organized parallel to one another across the entire leaf blade or lamina.

Reticulate Venation -

When veins (or nerves) are distributed in a network or web-like pattern over the entire leaf blade or lamina, this is known as reticulate venation.

Venation is the arrangement of veins on the leaf blade. Veins are composed of vascular tissues that are essential for the delivery of food and water. The veins of a leaf connect the blade to the petiole and the petiole to the stem.

Reticulate, parallel, and furcate venation are its three subtypes. 

Veins create the network-like structure in reticulate venation. For instance, dicot plant leaves.

Parallel venation is characterized by the emergence of veins from the mid-rib. For instance, monocot plant leaves. 

Veins create dichotomous branching in furcate venation. For instance, fern leaves. 

The two principal vascular tissues in leaf veins are xylem and phloem. Xylem is vital for the movement of water and soluble ions into the leaf, while phloem is important for the transport of carbohydrates (produced by photosynthesis) from the leaf to the rest of the plant.