ROOTS AND THEIR FUNCTIONS 147 



closed in a thin layer of water that contains from 0.1 to 1.0 per cent of 

 various dissolved substances, including those essential to plants. 



Substances taken in by the roots. Aside from carbon dioxide and 

 oxygen, which are discussed later, the various inorganic substances upon 

 which plant metabolism is dependent are obtained from the soil. Water 

 enters largely into the composition of all protoplasm and cell products. 

 It is one of the two substances from which the basic plant food glucose 

 is manufactured. It carries in solution the other materials used by plants. 

 Lastly, it maintains the internal pressure (turgor) within the cells of the 

 plant, which is responsible for much of the stiffness and strength of the 

 plant body and furnishes a mechanism for the movement of various parts 

 of the plant. The very large amounts of water used by plants are almost 

 wholly obtained from the soil, with some minor exceptions (aquatic 

 plants, epiphytes, or "air plants," parasites, and plants living in other 

 unusual situations). 



Besides the elements contained in carbon dioxide and water, plants 

 require relatively large amounts of certain others for the synthesis of 

 fats and proteins from glucose and its derivatives and for the building 

 of protoplasm and cell products. These essential elements include phos- 

 phorus, potassium, calcium, sulfur, magnesium, and nitrogen. All of these 

 are absorbed from the soil in the form of dissolved salts — nitrates, phos- 

 phates, sulfates, and ammonium salts. Phosphorus is a constituent of 

 certain fats and of nucleoproteins and is concerned in carbohydrate 

 transformations and in respiration. Potassium is in some way necessary 

 for the formation of sugar and starch, though how is not clear. Sulfur is a 

 component of many plant compounds, including proteins, and has some- 

 thing to do with the formation of chlorophyll. Calcium is essential for 

 the formation of the initial plate of new cell walls, and plays other physio- 

 logical roles. Magnesium is a constituent of chlorophyll and is therefore 

 indispensable to green plants; it also has other functions. Nitrogen is 

 essential for the synthesis of proteins and fats. 



Besides the above, plants require certain other trace elements in minute 

 amounts. Those known to be essential are iron, manganese, boron, cop- 

 per, zinc, and molybdenum. Although several thousand times as much 

 potassium as manganese may be utilized in the growth of a given plant, 

 the manganese is just as important to the normal existence of the plant 

 as the potassium. The significance of these trace elements has been 

 realized only in the last half century, and especially since 1923, when 

 boron was found to be essential to development of the broad bean, and 

 1927, when a group of Florida investigators found that addition of copper 

 to unproductive Everglades' soils rendered them valuable for truck 

 growing. 



Many formerly obscure plant diseases have been traced to deficiency 



