34 GROWTH OF THE PLANT 



bonaceous matter is being formed, the starch is acted upon 

 by amylases and is changed probably to dextrose. It is 

 thus passed from cell to cell by diffusion through the 

 cell walls and plasmatic linings until it passes out of the leaf 

 into the main conducting channels of the plant. These 

 channels are the so-called sieve tubes located outside of the 

 trachea?, or passages for the upward current of water and 

 plant food. 



Dextrose moves to that part of the plant needing new 

 material, such as newly expanding leaves, growing tips, 

 flowers, or roots. At these points other changes take place 

 in the dextrose. It may be reconverted into starch for 

 storage, or into cellulose for cell walls, or into oils, or into 

 any other of the numerous plant compounds. 



25. Amount of Carbohydrate Synthesized. — Averaging the 

 results of several experiments, it can be said that during 

 daylight on a bright day one square meter of leaf surface 

 manufactures about one gram of carbohydrate material in 

 one hour. For an acre of corn about the time of tasseling, 

 there is manufactured about 170 pounds of carbohydrate 

 in one day. 



26. Respiration. — As has been shown, the manufacture 

 of carbohydrates is brought about by the energy of light 

 waves, and the process is called photosynthesis. The manu- 

 facture of all other products is brought about by the energy 

 released on the oxidation of plant material, and the process 

 is called chemosynthesis. Oxidation or respiration is common 

 to the growing plant just as it is to the germinating seed, 

 although not to so great an extent. It is dependent on the 

 activity of oxidases in the presence of oxygen. This results 

 in the production of carbon dioxide and water, and a con- 

 sequent loss of plant substance. In other words the leaves 

 of plants are not only taking in carbon dioxide through their 

 stomata, making carbohydrates with water and giving off 

 oxygen, but they are also absorbing oxygen, oxidizing car- 

 bohydrates, producing water, and giving off carbon dioxide. 

 The gain of plant substance through photosynthesis, 

 however, is much greater than the loss through respiration, 

 for it is obvious that a growing plant gains in weight of dry 



