Chlorophyll

Chlorophyll is the pigment that gives all plants their green colour. It is sometimes called the blood of plants because of its similarity to haemoglobin - like haemoglobin, it can carry oxygen to other cells in the body.
Young cannabis plants contain the most chlorophyll and are therefore a great food supplement, similar to green barley or spirulina. However, a high chlorophyll content is not desirable when smoking cannabis - chlorophyll contains magnesium, which is responsible for the pungent, acrid smoke when lit.