[Chap. XXI INTERRELATIONS OF THE PARTS OF A PLANT 193 



to proteins. The tops do not grow well because there is a deficiency of 

 protein in them. Sugar and starch accumulate there and the sugar moves 

 downward and the roots become well supplied with it. The nitrates that 

 enter the roots are used there in protein synthesis, and as a consequence 

 the roots grow relatively large in proportion to the tops. 



Chemical analyses also show that the tops have a larger proportion 

 of protein when the supply of nitrates in the soil is high. Under these 

 conditions the tops grow well. A relatively greater amount of the sugar 

 made in the chlorenchyma is used directly in the growth of tops, and 

 consequently the roots are less well supplied with sugar and their rela- 

 tive growth is less.^ 



Hormones and interrelations. Having seen how this one example of 

 growth can be interpreted upon the basis of foods without assuming any 

 purposeful acts by a part of the plant, we may now ask whether the 

 growth of roots is dependent upon the tops only for sugar. It is possible 

 that their growth may be dependent upon the tops also for minute 

 amounts of certain hormones (or auxins), vitamins, and amino acids, 

 or for certain precursors of these substances. This question was asked 

 many years ago, and it has been answered in the affirmative and the 

 negative many times. Recently, however, there has been a concerted 

 effort by many workers to decide it on the basis of critical experiments. 



A brief account of one type of these experiments will indicate some 

 interesting features of the general problem. Several attempts have been 

 made to place small pieces of roots in solutions of certain materials in 

 which they would continue to grow entirely isolated from the rest of the 

 plant. These efforts were finally successful. One investigator put a small 

 piece (1 cm. long) of the tip of a tomato root in 50 cc. of water in a 

 sealed flask which also contained: 



1,000.000 mg. of sucrose 



17.300 mg. of essential inorganic salts 

 0.005 mg. of brewer's yeast extract. 



The pieces of root grew in this culture. At the end of one week a small 

 tip of this root was transferred to a fresh solution, and so on week after 

 week. These pieces of roots continued to grow after each transfer with- 

 out any decline in the rate of growth with time. But if the brewer's yeast 

 was omitted the root ceased to grow after two or three weeks. 



What is there in this minute amount of brewer's yeast that is so im- 



^ The other examples listed are left for class discussion. 



