102 PHYSIOLOGY OF NUTRITION 



Summary 



i. Cultures in Artificial Media. — The essential chemical elements for plants in 

 general are: Carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, 

 calcium, magnesium, and iron (C, H, O, N, S, P, K, Ca, Mg, Fe). Carbon and oxygen 

 enter ordinary green plants as carbon dioxide (C0 2 ), while hydrogen and oxygen 

 enter as water (H 2 0) . As already seen, these two compounds are decomposed in chlo- 

 rophyll-bearing cells, by the action of sunlight, forming carbohydrates ([CH 2 0]„) 

 and free oxygen. From carbohydrates and other substances the plant cells form the 

 many different organic compounds found in the plant body. Tissues without chloro- 

 phyll must absorb their carbohydrates (and often many other organic compounds from 

 their surroundings, including the green tissues of the same plant. As has also been, 

 seen, nitrogen enters the ordinary plant mainly as nitrates (sometimes as nitrites, 

 ammonium salts, or organic nitrogenous substances), and these become combined with 

 carbohydrates, etc., to form many of the most complex substances occurring in plants. 

 When plants are completely burned all of the carbon, hydrogen, oxygen, and nitro- 

 gen are given off as gases, but there remain small amounts of many other essential 

 and non-essential elements in the form of incombustible ash. The total ash of ordinary 

 plants constitutes only about 5 per cent, of the total dry weight, or about 0.02 per 

 cent, of the green weight. The other essential elements (S, P, K, Ca, Mg, Fe) of the 

 ash are absorbed by ordinary plants from the soil, just as are water and nitrates, and 

 the supply is in the form of inorganic salts: mainly nitrates (N0 3 ), sulphates (S0 4 ), 

 and phosphates (P0 4 ), of potassium, calcium, magnesium, and iron. 



The elements absorbed through the roots may be studied by artificially controlled 

 cultures in water solutions or in pure quartz sand, etc., the latter of course containing 

 water solution in its interstices. Numerous different solutions have been tested by 

 many workers. A very good medium for solution cultures may be prepared with cal- 

 cium nitrate [Ca(N0 3 ) 2 ], mono- or di-potassium phosphate, (KH 2 P0 4 or K 2 HP0 4 ), 

 and magnesium sulphate (MgS0 4 ), about seven thousandths of a gram-molecule (the 

 molecular weight expressed in grams) of each salt, all dissolved together in a liter of 

 water, with addition of a very small amount (about 3 mg.) of an iron salt such as 

 ferrous sulphate (FeS0 4 .7H 2 0). This is one of Shive's nutrient solutions. Three 

 single-salt solutions (with trace of an iron salt) may be used in rotation, if concen- 

 trations and time periods are properly chosen. 



2. Importance of Essential Ash-constituents.— Little is known as to just how the 

 small amounts of essential ash constituents are used in the plant, but all must be 

 supplied. Sulphur occurs in proteins, phosphorus in nucleins (a special group of 

 protein-like substances), magnesium occurs in chlorophyll, and iron is essential for 

 the formation of chlorophyll. 



3. Importance of Non-essential Ash-constituents. — Although plants grow well 

 with only the essential elements supplied, yet they generally contain many non-essen- 

 tial elements, and these are not without influence upon growth and development when 

 they are present in the right amounts. Grasses accumulate silica in the epidermis and 

 are thus more or less protected, from fungi, etc., by a glassy layer on the exterior. 



4. Ash Analysis of Plants. — The chemical analysis of the ash of a plant shows what 

 elements are present and in what proportions they occur. Different species differ in 

 these respects, and also in the amount of total ash per unit of weight, etc. The nature 

 of the soil influences the ash content of the plant. Different parts of the same indi- 

 vidual plant differ in ash content. Leaves are generally richer in ash than stems and 

 roots. The ash content alters with the age of the organ or tissue. 



