Polarity is a basic phenomenon in plants. Induction of polarity by blue light is demonstrated on cellular level in side branch formation of the alga Vaucheria (Siphonales) and in the germination of the spermatized eggs of the brown alga Pelvetia. Polarity, once established in a cell persists during the whole lifetime of a plant. Expression of polarity is illustrated in roots and in under water leaves of higher plants; in the case of the leave it is characterized by the flux of protons from under to upper side. The influence of proton flow in leave and root is molecularly visualised using computer animations.