382 ANNUAL, EEPORT SMITHSONIAN INSTITUTION, 19 31 



by the plant during its growth. Van Helmont had neglected to con- 

 sider the air as a very important source of material for plant food. 

 Since his day, scientists have discovered that plants absorb carbon 

 dioxide from the air, and, by means of the energy of sunlight, unite 

 it with water to form sugars and starch. This process is known as 

 photosynthesis. From these manufactured substances the plants 

 build more complicated food materials which are used in their 

 growth and reproduction. 



The ash that remains after the water and combustible materials 

 have been driven off by heat should not be overlooked. Although 

 the amount of ash is small in comparison to the other materials mak- 

 ing up the plant structure, it contains several ver}^ important mineral 

 elements that are vital to the growth and vigor of the plant. The 

 more commonly known elements are calcium, phosphorus, potassium, 

 magnesium, and sulphur. These are some of the elements that form 

 the essential ingredients of fertilizers. More than 30 elements have 

 been found in the ash of plants. The amounts of some elements such 

 as chloride, sodium silicon, and iodine vary greatly in different types 

 of plants. Even traces of zinc, tin, lead, silver, and copper have been 

 found. Scientists do not at present know if all these elements are 

 necessary for the growth of plants, but they are finding that minute 

 traces of some elements, such as boron, manganese, and zinc are just 

 as essential as those usually mixed to form commercial fertilizers. 



As early as 600 B. C. it was thought that plants obtained all their 

 food from water. It is not water alone that the plant takes through 

 the roots, but the mineral elements dissolved in it. Even to-day many 

 people are surprised to hear that solid particles of fertilizers are not 

 absorbed by plant roots. These mineral substances must first be dis- 

 solved in the soil water before they can pass into the plant through 

 the delicate root membranes. In fact the solid soil may be dispensed 

 with and the roots surrounded by liquid onl3\ However simple the 

 technique of growing land plants with their roots in water may now 

 be, it was not until 1699 that there were any published records of 

 such plants grown in water cultures. Woodward grew spearmint, 

 potatoes, and vetch in water from a conduit, in river and spring 

 water, as well as in rain and distilled water. His purpose was to 

 determine whether the water itself or what it contained in the way 

 of dissolved matter w^as the nutrient material used by plants. He 

 concluded that the water was the carrier of the necessary " terrestrial 

 matter." 



The next important step was for man to add artificial terrestrial 

 matter to pure or distilled water and thereby determine which ele- 

 ments were essential to good plant growth and which were not. Then 

 followed a series of experiments by plant investigators to determine 



