384 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 31 



When the crocks were new the plants obtained a sufficient supply of 

 boron from the glaze. Later this element was more difficult for the 

 plants to obtain as the available supply became exhausted. 



A simple experiment clearly brought out these facts. The same 

 crocks were again used, as well as sand from the same source used 

 in the earlier experiments. However, a very small amount of boron 

 was dissolved in the solution used for watering half the plants. 

 This amount was almost inconceivably small, being one part of 

 boron to 2,000,000 parts of the solution. The potato plants had 

 little difficulty in detecting and using this minute quantity of boron. 

 The difference in growth is clearly seen in Plate 1. Imagine to 

 what extent a man would be nourished by eating pea soup with one 

 pea to each 132 gallons of water. Yet there are minute quantities 

 of certain substances just as important to man as boron is to plants. 

 Certain glands manufacture traces of interesting chemical com- 

 pounds called hormones which stimulate or retard the growth of 

 animal tissue. The discovery of iodine in the thyroid was followed 

 by the isolation of thyroxine, a stable iodine-containing hormone. 



Too great quantities of certain elements in the soil are just as 

 harmful to plants as too small amounts. Slight differences fre- 

 quently bring about enormous growth responses. To obtain two or 

 more pots of soil that contain exactly the same amounts of all chem- 

 ical elements affecting plant growth is a herculean task. Even if 

 two such pots of soil were obtained it would be even more difficult 

 to determinate accurately the amounts of all the chemicals used by 

 the plants. It is hence easy to see that soil cultures are not very 

 good media for growing plants in accurately controlled experiments. 

 For these reasons and for the simplicity of handling and controlling 

 the elements, water cultures are being used more and more where 

 exact information of the plant's growth and behavior is desired. 



Water-culture experiments, or those in which plants are grown 

 in mineral nutrient solutions made by dissolving pure chemicals in 

 distilled water, have furnished science a means of distinguishing the 

 useful or essential plant-food elements from the useless or nonessen- 

 tial. It is true that chemical analysis demonstrates the presence of 

 many elements in plant tissues. It is poor reasoning, however, to 

 claim they are all essential. Plants as well as animals take into their 

 tissues certain nonessential substances along with the essential. 

 The late Prof. Cyrus Hopkins gave to his students a mnemonic sys- 

 tem whereby they could easily remember the then supposed essential 

 elements for plant growth. It read " C. Hopkins cafe, mighty 

 good." When expressed as chemical symbols it appears as follows : 

 C. HOPKNS, Ca Fe, Mg, which represent the elements carbon, hy- 

 drogen, oxygen, phosphorus, potassium, nitrogen, sulphur, calcium, 



