STRUCTURE AND PHYSIOLOGY OF PLANTS 17 



crops must be in soils supplied with oxygen. If there is too 

 much standing water in a field the air is excluded from the 

 soil and crops do not thrive. The leaves of plants turn 

 yellowish and show an unhealthy condition if the soil is not 

 supplied with air. This may be noticed in the wet parts of 

 a cornfield. If the soil is well drained, the surplus water 

 gets away and air is drawn into the soil. 



Exercise. — Need of Air for Germination. — Put a dozen 

 or more beans or kernels of corn in a bottle nearly full of 

 water. In another bottle place a dozen of the same kind, 

 thoroughly wet but not submerged in water. Cover both 

 bottles loosely to prevent loss of water. Put both in a 

 warm place and watch results for a week or so. The water 

 in the first bottle excludes the air from the seeds. If they 

 do not sprout it will be for want of air. When crops are 

 planted in low, wet places in fields the seeds refuse to grow 

 for lack of air. 



Exercise. — Need of Air in Soil. — Two cans of soil may 

 be planted alike with the beans or corn. Have one can 

 watertight at the bottom, and the other with plenty of 

 holes through the bottom for drainage. When the plants 

 have grown a few inches high fill the undrained can with 

 water to the top of the soil. Notice the effect on the plants 

 in that can. Compare with the others having air and moist- 

 ure both in the soil. 



Purpose of Flowers. — The main aim of life for all plants 



is to produce seed. To do this a flower must be formed. 



The form and structure of flowers are quite variable, as seen 



when the strawberry is compared with the lady-slipper 



and the apple. 



A perfect flower has two different kinds of essential 

 organs, the pistils and the stamens. The stamens bear 

 pollen, which is carried to, or falls upon, the pistil. This 

 fertilizes the pistil and enables it to produce the seed. The 

 seed or seeds of most plants are borne in the lower part of 

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