Plant Qrowth'Suhstances 



highly-manured greenhouse soils, such as is treated by means 

 of partial sterilization: growth-substances of the indole and 

 other groups can be toxic in too great a concentration. 



In a letter to Nature under the title "Role of Heteroauxones 

 in Legume Nodule formation, Beneficial Host Effects of 

 Nodules, and Soil Fertility", Link (1937) has suggested that 

 "the beneficial effects of suitable concentrations of (3-indole- 

 acetic acid and other auxones may account in part for the 

 characteristically beneficial effects of: (i) nodules for some 

 host plants; (2) green manuring with nodule-bearing plants; 

 (3) fertilizing with manures rich in dung and urine, or with 

 compost; (4) humus soil; and (5) mycorrhizal fungi for some 

 host plants." 



This claim has been anticipated to some extent by other 

 workers^, and notably at some length in relation to green 

 manures by Schreiner, Reed, and Skinner in 1907. Whereas 

 unchanged tyrosine was found by Schreiner, Reed and 

 Skinner (1907) to be toxic to young wheat seedlings, a 

 tyrosine solution, after having become discoloured on stand- 

 ing, produced excellent growth of young wheat seedlings. 

 The colour of this old tyrosine solution resembled that of 

 manure extract, and the wheat plants grown in it resembled 

 plants grown in manure extract. The darkening was possibly 

 due to the oxidation of tyrosine to homogentisic acid (2:5- 

 dihydroxyphenyl-acetic acid) which gave rise to further 

 oxidation compounds of dark colour. The effect on plants 

 of homogentisic acid has not otherwise been investigated. 

 The chemical relationships of this substance have received 

 attention owing to the occurrence of homogentisic acid in the 

 urine of people subject to the rare condition known as "alcap- 

 tonuria" (see p. 74). 



^ In 1935 the author wrote: "One of the most remarkable effects of the 

 leguminous crop, whether in mixture or in rotation, is its apparent ability to 

 supplement animal manures. In peninsular Indian practice (the best-studied 

 case) it would seem that legumes grown in mixture fill the place of animal 

 manures. It appears unlikely that this ability is due solely to the nutrient 

 nitrogen compounds supplied by the legumes, and it is probable that 

 leguminous plants everywhere make a definitely biological contribution to 

 the fertility of soil." — Hugh Nicol, "Mixed cropping in primitive agricul- 

 ture," Empire Journ. Exper. Agric, 1935, 3> i8q. 



6+ 



