“Af Klint’s many notebooks, which expound and elaborate on the sometimes enigmatic symbology of her work, offer a codex to this system,” Daniel Birnbaum and Emma Enderby write in their essay Painting the Unseen, in the catalogue.... “Logarithmic spirals and tendrils represent evolution; the letter ‘U’ stands for the spiritual world, opposing ‘W’ for matter; the ancient vesica piscis (the intersection of two overlapping discs) signifies its traditional theme of unity, creation and the inviolability of geometry. The color yellow and roses stand for masculinity; the color blue and lilies denote femininity.” With this complex language of colors and symbols, Af Klint's work becomes instantly more readable. It was a pursuit for “oneness”, the pair continue: in it, green represents the merging of yellow and blue, or male and female, or good and evil, becoming the ultimate aim. (http://www.anothermag.com/art-photography/8490/decoding-the-spiritual-symbolism-of-artist-hilma-af-klint)