66 



PLANT GROWTH AND NUTRITION 



at once. The surer test of limewater shows the presence of car- 

 bon dioxide in the jar. The carbon of the foodstuffs of the pea 



united with the oxygen 

 of the air, forming car- 

 bon dioxide. Growth 

 stopped as soon as the 

 oxygen was exhausted. 

 The presence of carbon 

 dioxide in the jar is an 

 indication that a very 

 important process which 

 we associate with animals 

 rather than plants, that 

 of respiration, is taking 

 place. The seed, in order 

 to release the energy 

 locked up in its food 

 supply, must have oxy- 

 gen, so that the oxida- 

 tion of the food may take 

 place. Hence a constant supply of fresh air is an important factor 

 in germination. It is important that air should penetrate between 

 the grains of soil around a seed. The frequent stirring of the soil 

 enables the air to reach the seed. Air also acts 

 upon some materials in the soil and puts them 

 in a form that the germinating seed can use. 

 This necessity for oxygen shows us at least 

 one reason why the farmer plows and harrows 

 a field and one important use of the earthworm. 

 Explain. 



Structure of a Grain of Corn. - - Examination 

 of a well-soaked grain of corn discloses a difference 

 in the two flat sides of the grain. A light-colored 

 area found on one surface marks the position of 

 the embryo ; the rest of the grain contains the 

 food supply. The interesting thing to remember here is that the 

 food supply is outside of the embryo. 



Experiment that shows the necessity for air in 

 germination. 



A grain of corn 

 cut lengthwise. 

 C, cotyledon; 

 E, endosperm; 

 H, hypocotyl ; 

 P, plumule. 



