142 Plant Physiology 



magnesium. Such substances as lime, silicon, and chlorine 

 do not seem to move appreciably. According to Arendt 

 1000 oat plants contained in the various periods of growth 

 the quantities of phosphoric acid given in the preceding 

 table, expressed in grams. 



77. Water cultures. ' From a study of the nutrient 

 requirements of plants in soils, or even in sand cultures, it 

 is not possible to arrive at a definite conclusion respecting 

 the elements needed by plants through the soil solution. 

 For this purpose water cultures are required, and such 

 cultures have been employed for more than half a century 

 in the study of plant nutrition and other physiological 

 relations. Relatively simple experiments afford the chief 

 fundamental facts. Many plants lend themselves to 

 water-culture experiments ; in fact all cereals, peas, beans, 

 buckwheat, and many other crop plants may be employed, 

 in spite of the unusual conditions to which the roots are 

 subjected. 



The seed represents a considerable accumulation of 

 necessary mineral nutrients as well as organic foodstuffs, 

 and if so supported that the roots may grow in a vessel of 

 distilled water, this supply of nutrients alone may support 

 a strong growth for one or two weeks. If peas or beans are 

 employed and the cotyledons are cut off as soon as the plu- 

 mule is well developed, the growth in distilled water will be 

 very slight. Ordinary well water, or the seepage water from 

 a tile drain, used as a culture medium, and frequently re- 

 newed, affords a vigorous growth. 



i A water culture containing as soluble salts the elements 

 nitrogen, phosphorus, potassium, calcium, magnesium, 

 sulfur, and iron will afford more or less perfect growth. 



