There are two main sources of signals that plant development responds to: external (or environmental) The sessile nature of plants demands that their development is continuous throughout a lifespan and remains plastic to adapt to changes in the environment. Light quality and quantity is a significant environmental factor affecting plant development. Phytochrome plays a central role as a light receptor that regulates cell development: internal. Multicellularity confers the selective advantage of a more efficient and adaptive functioning of the whole organism through a division of labour of its component parts. Specialisation that allows division of labour leads to a dependency on the entire organism and hence a need for coordination between its component parts. Development of specialized structures to perform specific functions requires coordination in both space and time. Internal signals that are known to coordinate plant development are: plant hormones, nutrients (sugars, nitrogen compounds), and macromolecules (dna, rna and proteins).