Investigation, of which the following is some account, have been undertaken with a view to being of assistance in the interpretation of the phenomena observed in the interpretation of the phenomena observed in the inheritance of flower-colour. An attempt has been made to classify, necessity roughly, the pigments, more especially those soluble in water, found in flowering plants, and at the same time to ascertain whether there is any connection between the genetic bahaviour of pigments and their chemical reactions and constitution. On the basis of this classification it was thought that, at some future time, further investigation might be carried out in greater detail among that various classes of pigments. This account deals more with yellow pigments than with red; attention was first directed to the yellow group, because of a certain correlation between reds and yellows observed in the inheritance of flower-colour in Antirrhinum majus , the relationship suggesting that a greater knowledge of yellows might be useful in classifying the reds. No detailed examination has been made of any one pigment, but merely a general survey of the colouring matters of genera from various natural orders.