58 PLANT GROWTH AND NUTRITION 



Why did not the seeds in the covered jar germinate? To 

 answer this question, let us carefully remove the stopper from the 

 closed jar and insert a lighted candle. The candle goes out at 

 once. The surer test of limewater shows the presence of carbon 

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

 with the oxygen of the air, forming carbon 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 with plants, 

 that of respiration, is taking place. The seed, in order to release 

 the energy locked up in its food supply, must have oxygen, so that 

 the oxidation 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. Frequent stirring of the soil makes it easier for air to reach 

 the seed. 



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 po- 

 sition of the embryo ; the rest of the grain contains the food sup- 

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

 supply is outside of the embryo. 



^ A grain cut lengthwise perpendicular to 



^'^ the flat side and then dipped in weak iodine 

 Cotyledon shows two distinct parts, an area containing 

 Plumule considerable starch, the en'dosperm, and the 

 embryo or young plant. Careful inspection 

 shows the hypocotyl and plumule (the latter 

 pointing toward the free end of the grain) 

 Hypocotyl and a part surrounding them, the single coty- 



ledon (see figure). Here again we have an 

 Section of corn grain, ^^ample of a fitting for future needs, for in 

 this fruit the one seed has at hand all the food material necessary 

 for rapid growth, although the food is here outside the embryo. 



Endosperm the Food Supply of Corn. — We find that the one 

 cotyledon of the corn grain does not serve the same purpose to 

 the young plant as do the two cotyledons of the bean. Although 

 we find a little starch in the corn cotyledon, still it is evident from 



