PHYSIOLOGY OF CELLS 40 



used by the plants in their uncombined form, but as compounds. The 

 carbon, except in plants that get material already elaborated bj^ other 

 organisms, is derived from the carbon dioxide of the air. Hydrogen 

 and oxj^gen are utilized in the form of water, and oxygen is also secured 

 uncombined from the air or from solution in water, and combined in 

 many salts and oxides. Nitrogen is utilized by most plants in the form 

 of nitrates. Lower plants, as fungi and certain algse, and some of the 

 carnivorous plants secure nitrogen in other forms. One of the chief 

 sources of nitrogen is ammonia (NH3) which is one of the products of 

 the bacterial decomposition of organic material, that is, the bodies of 

 plants or animals, or animal excrement. Ammonia is not, however, 

 used as ammonia by green plants. The nitrogen of ammonia is made 

 available to green plants through the action of certain bacteria which 

 transform the ammonia into nitri tes while other bacteria transform the 

 nitrites in to nitr ates. Asjiitrates the nitrogen can be utilized by green 

 plants. In addition to these bacteria certain other species, of which 

 Azotobacter is an example, are capable of fixing free nitrogen from the 

 air, that is, of converting it into chemical compounds, and thus making 

 it available for use, not only by these bacteria but also by green plants. 

 Certain species of nitrogen-fixing bacteria live in symbiotic relation with 

 leguminous plants, as beans and clover, forming nodules on their roots. 

 These bacteria fix more nitrogen than they can use for themselves and 

 this is available for the green plant. Sulphur is taken into the plants as 

 sulphates; phosphorus in the form of phosphates; potassium, magne- 

 sium, calcium, iron and other elements in the form of various salts in 

 solution. 



Photosynthesis. — Carbon is an essential part of all living matter. 

 Its source for green plants is the carbon dioxide of the air, or for sub- 

 merged plants the carbon dioxide dissolved in the water. The carbon 

 dioxide is derived from the oxidation of organic matter. Carbon dioxide 

 dissolved in water diffuses through the outer layer of protoplasm of the 

 plant cell and is taken up by the chloroplasts, the green plastids of the 

 cell. In these chloroplasts is a green substance known as chlorophyll, 

 with two accompanying yellow substances carotin and xanthophyll. The 

 chloroplasts are the factories in which the carbon dioxide is transformed 

 into carbohydrates. The raw materials used are carbon dioxide and 

 water, and the energy for the operation is derived from light. For the 

 manufacture of carbohydrates four requirements must be met, namely, 

 the factory, the two kinds of raw material, and the energy. There can 

 be no carbohydrate manufacture in the dark, nor without carbon dioxide 

 and water, nor (with a few exceptions) in the absence of chlorophyll. 

 It is known that several enzymes (see below) also play a part in the 

 production of carbohydrates. It has been shown that the red, orange, 

 and 3'ellow rays of the spectrum are most effective in the process, while 



