Phytomorphology (or Plant Morphology) is the study of the physical form and external structure of plants. This would include leaves and leaf structures.
phyto- (derived from the Ancient Greek for "plant:" φυτόν; phutón) is a prefix defined as "that which pertains to or is derived from plants," and morphology, which is a scientific study of form and structure; in biology this would be the form and structure of animals and plants.
"As a scientific discipline, plant morphology is 211 [years] old, originated by Goethe in 1790." (From: "The science of plant morphology: definition, history, and role in modern biology" by Donald R. Kaplan; American Journal of Botany).
If you truly want to get more specific, you would just refer to your study of leaves alone as "Leaf Morphology," or maybe perhaps "Phyllomorphology" (Phyllo- being the greek for "leaf:" φύλλο).
Phyllomorphology as opposed to Phyllotaxonomy (the scientific identification, naming, and classification of living things: specifically leaves).