Extension growth of the hypocotyl of de-etiolated cucumber seedlings is inhibited by blue and red light, the latter operating through the phytochrome system. The major part of the inhibition due to red light is mediated through the cotyledons which transmit the effect to the hypocotyl. On the other hand, blue light acts directly upon the hypocotyl, the cotyledons apparently not being involved. Thus, there is a morphological separation of spectral sensitivity. The role of the cotyledons is evident only in de-etiolated seedlings. Etiolated plants also differ in being sensitive to prolonged far-red light whose effect becomes less apparent as seedlings age. De-etiolation seems to abolish sensitivity to far-red.