Which substance makes plants green?

Chlorophyll
Chlorophyll (also chlorophyl) is a term used for several closely related green pigments found in cyanobacteria and the chloroplasts of algae and plants.

Its name is derived from the Greek words χλωρός, chloros ("green") and φύλλον, phyllon ("leaf").

Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to absorb energy from light.

Chlorophyll absorbs light most strongly in the blue portion of the electromagnetic spectrum, followed by the red portion.

Conversely, it is a poor absorber of green and near-green portions of the spectrum which it reflects, hence the green color of chlorophyll-containing tissues.

Chlorophyll was first isolated and named by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817.

Read more...https://en.wikipedia.org/wiki/Chlorophyll