66 PHYTOHORMONES 



on growing stems, young leaves on growing petioles, inflo- 

 rescences or flowers on growing flower-stalks, and coleoptile 

 tips. Now we know that in young seedlings the last stage in 

 the production of auxins, i.e. the transformation of the pre- 

 cursor into the active form, can take place in darkness 

 (Skoog, 1937). Since we must conclude, from the general 

 agreement between the experiments on plants other than 

 seedlings, that auxin production in such plants takes place 

 only as a result of the action of light, it follows that light is 

 necessary for the formation of the auxin precursor. Light is, 

 of course, necessary for the formation of all organic sub- 

 stances in the plant, through its part in producing carbo- 

 hydrate ; this carbohydrate may afterwards undergo changes 

 to other substances, such as fats and proteins, but without 

 the intervention of light. Now the fact that auxin produc- 

 tion in Lupinus and Raphanus seedlings may become very 

 low in darkness, while the cotyledons are still full of carbo- 

 hydrate and other reserves, shows that the effect of light 

 on the formation of the precursor cannot be simply through 

 the photosynthetic production of carbohydrate. There 

 must, on the other hand, be a definite light-sensitive reac- 

 tion involved in the formation of the precursor. Along 

 completely different lines, Gregory (1928) came to a similar 

 conclusion with regard to the effect of light on leaf growth; 

 although leaf growth is directly dependent on light its Qio 

 is different from the Qio of photosynthesis, and hence he 

 concludes that ''a master photochemical reaction, independ- 

 ent of carbon assimilation, leads to the formation of a sub- 

 stance directly involved in leaf expansion." Whether auxin 

 itself plays any part in leaf expansion has not yet, however, 

 been shown. There is evidence that it is connected with the 

 elongation of the veins, but the mesophyll is not influenced 

 at all by auxin application. 



The exceptions to the formation of auxin as a result of 

 the action of light are all provided by seedlings. In these, 

 as we have seen, it is almost certainly formed from an 

 inactive precursor which is stored in the seed. It is impor- 



