Aspen leaf. Close-up of a section through a leaf of a bigtooth aspen tree (Populus grandidentata). The veins are seen as a pale network, whilst the main vein (midrib, thick) runs through center. The vein network carries water (in xylem tissue) and sugar nutrients (in phloem tissue) throughout the leaf. A leaf is the main energy-gathering organ of a green plant. It traps the energy of sunlight by using the pigment chlorophyll as part of a process called photosynthesis. The chlorophyll gives the leaf its green color. The bigtooth aspen tree is found in northeastern North America.