The term 'RIB OF A LEAF' can be solved by the four-letter word 'VEIN'. A VEIN is a vascular structure in a leaf that carries water and nutrients from the stem to the rest of the leaf. Veins are typically located on the underside of the leaf and form a network of veins that branch out from the midrib, which is the main VEIN running down the center of the leaf. The veins provide support to the leaf and help it to absorb sunlight for photosynthesis.