16 THE NATURE AND WORK OF PLANTS 



lacking in solid matter which might be used as 

 food by the plant. 



Now boil a quart of water from a stream or 

 pond in the same dish until it has all passed away 

 in the form of vapor. A gray coating remains on 

 the bottom of the dish, consisting of mineral salts, 

 which may be taken up by the plant, and which 

 forms the ash when it is burned. This is identical 

 with the "lime" which is deposited on the inside 

 of tea-kettles in which hard water is boiled. 



15. Elements found in plants. — If a complete 

 chemical analysis were made of the plant, it would 

 be found that it had selected sulphur, phosphorus, 

 magnesium, calcium, potassium, sodium, and iron 

 from the minerals present and had taken these up 

 in certain proportions most useful to it. Besides 

 these, it also gets nitrogen, chlorine, carhon, hydro- 

 gen, and oxygen from the soil and air. Many species 

 take up quantities of silicon, as did the scouring rush. 

 The seaweeds use iodine and bromine, and still other 

 elements are taken up by a few forms, and may 

 or may not serve as food. Thus many of the grasses 

 and sedges do this, and some of them use the 

 mineral to give a sharp saw-toothed edge to their 



