Abscission:  the dropping of leaves from a deciduous tree usually following the formation of an abscission zone and a color change.

Aphelion: The point that occurs when the earth is furthest away from the sun.

Auxin: a plant hormone, also called IAA, responsible for cell elongation, fruit development, and formation of adventitious roots.

Carotene: yellow-orange colored, fat-soluble plant pigments found in chloroplasts and chromoplasts that aid in absorption of light for photosynthesis.

Chlorophyll: a group of green colored pigments a and b that are the primary pigments involved in the absorption of light for photosynthesis. 

Compound leaf: a leaf, made of numerous leaflets, that is connected to the stem through a petiole.

Ethylene: a plant hormone responsible for fruit ripening and abscission zone formation.

Morphology: the physical form and structure of animal and plants.

Perihelion: The point when the earth is closest to the sun.

Petiole: a small stem-like structure that connects a leaf blade to a stem.

Photoperiod: a recurring cycle of light and dark periods in which the period length may vary.

Physiology: the organic and chemical processes of a living organism.

Plastid: a cytoplasmic organelle in a eukaryotic plant cell that contains pigments involved in photosynthesis.

Simple leaf: a leaf that consists of a single blade and is connected to the stem through a petiole.

Suberin: a waxy substance that is found in plant cells.

Vacuoles: a small cavity or space containing liquids found in the tissue of an organism or the cytoplasm of a cell.

Xanthophyll: yellow colored, fat-soluble plant pigments found in chloroplasts and chromoplasts that aid in absorption of light for photosynthesis.