PLANT HORMONES AND GROWTH 



problem of whether they arc causative of growth or not, tliat remains 

 unsettled. 



To some extent, the need for postulating these indirect mecha- 

 nisms arises from the difficulty of designing experiments which bear 

 upon the growth process per se, and which introduce the minimum of 

 extraneous considerations. In the study of vitamin action, it is usual 

 to employ vitamin-deficient animals, but it is not .so easy to obtain with 

 certainty plants which are deficient in auxin. The discussion of auxin 

 reserves in the preceding section brings out the difficulties involved. 

 Green plants in the light, rich in stored auxin, vitamins, and carbo- 

 hydrates, would appear to provide the worst type of experimental 

 material. But even embryos and dark-grown seedlings, though more 

 nearly ideal, and certainly capable of yielding more quantitative data, 

 have their reservoirs of auxin and food factors. It is evident that 

 isolated parts of plants, grown in the dark under controlled conditions 

 and free from storage organs, approach nearest to the rigid require- 

 ments of experimental material for the study of auxin action. With 

 this in view many studies have been made of the growth of isolated 

 sections, 3 to 5 mm. long, cut from etiolated oat coleoptiles. Such 

 sections will elongate to a small extent in water, and to a much greater 

 extent in auxin solution, the amount of elongation being proportional 

 to auxin concentration up to a maximum at about 10 mg. per liter. 

 Higher concentrations cause reduction in growth and are toxic. This 

 elongation is achieved at the expense of the stored carbohydrates in 

 the cells. If sugar is added to the solution, the elongation is very 

 greatly increased both in amount and in duration. Nitrogenous 

 substances have not been found to increase growth any further, though 

 some salts, particularly potassium chloride, cause a further increase. 

 Optimal concentrations are: sucrose, 1%; potassium chloride. A// 100; 

 and indoleacetic acid, 1-5 mg. per liter. These isolated sections pro- 

 vide a simple growing system which is as free from indirect controls 

 as can readily be envisaged. Growth is a function of temperature, of 

 the osmotic gradient, and of the concentrations of auxin, potassium 

 ions, and sugar. It is difficult to see how auxin could cause growth 

 here by mobilizing anything. 



Mainly with this material it has been shown that growth is 

 controlled by a specific process of respiration. It had been shown 

 many years earlier by Bonner that cyanide inhibits growth over the 



