SUMMARY 47 



The commercially valuable part of these crops differs with 

 the kind of plant, thus making any general discussion 

 impossible. For example, the apple, pear, squash, and pump- 

 kin are raised for the fleshy seed coverings; celery and rhubarb 

 for their thickened stems; the carrot and beet for their 

 thickened root; and spinach and lettuce for their leaves. 



49. Summary. — For the normal growth and development 

 of crop plants, a certain amount of plant food is required. 

 Plant food may be defined as a substance supplying one or 

 more elements necessary for normal plant activities. The 

 essential elements are ten in number, being carbon, hydrogen, 

 oxygen, phosphorus, potassium, nitrogen, sulphur, calcium, 

 iron, and magnesium. There are also used silicon, sodium, 

 and chlorine to a considerable extent. Three-fourths of a 

 green plant is water; the other fourth is mostly carbon and 

 oxygen with lesser amounts of hydrogen and the other 

 elements. 



The plant foods are absorbed as inorganic, water soluble 

 compounds only, except oxygen which is partly absorbed 

 in the elemental form. Carbon is absorbed as a gas, carbon 

 dioxide; hydrogen and oxygen, as water. The remaining 

 seven essential elements are absorbed as salts of their several 

 basic and acid radicles. 



The root hairs situated near the tip end of the growing 

 roots serve as ports of entry for the dissolved material. 

 These soluble compounds pass from cell to cell by osmosis 

 until they reach the trachea? where they are forced up 

 through the stem to the leaves which are the seat of manu- 

 facture of plant material. In the leaves there are chloro- 

 phyl bodies which absorb certain orange-red light waves 

 from the sun and change the radiant energy of the sun into 

 potential chemical energy of carbohydrate compounds. 

 Carbon dioxide and water are combined to form formal- 

 dehyde, then dextrose, and usually starch which is the 

 temporary storage form of manufactured organic material 

 in the leaf. 



For transport to various parts of the plant needing building 

 material, the starch is hvdrolvzed by enzvmes and water 

 to dextrose, and carried by osmosis to the needed point. 



