PHYSIOLOGY OF CORN PLANT 39 



right position; it moves in a direction exactly opposite to the 

 attractive force of gravity. If a tap-root be similarly experi- 

 mented upon, it wiR, on the contrary, continue its downward 

 movement until it places itself in a line with the direction of 

 the attraction ; a rhizome, however, under Hke circumstances, 

 would constantly maintain its growing apex in a horizontal 

 position. In these three experiments, the force of gravity is 

 exerted upon horizontal portions of plants. The physical condi- 

 tions are the same in each case, yet how entirely different the 

 results." 



The above phenomena are some of the manifestations of 

 " life." In the same way, hght, heat, moisture, and 

 other physical factors will act as a " stimulus " to living 

 plants, but the response is not always what would be ob- 

 tained with dead material, and it may be the opposite. 

 This fact should be kept in mind in dealing with living 

 plants. 



28. Stability of the plant. — A corn plant one inch in 

 diameter at the base may be 100 to 125 inches in height, 

 yet it will have a broad spread of leaf, bear a heavy ear, 

 and be able to maintain itself without breaking or falling 

 prostrate in a heavy wind. A rye plant bearing a heavy 

 head may be five hundred times as tall as the diameter of 

 its base. This rigidity of the plant body is necessary in 

 order that it may reach considerable height and expand its 

 leaves to light and air. Rigidity is due principally to 

 turgidity in the soft tissue or young plant, and to the 

 mechanical tissues in the older and stronger parts. 



29. Turgidity. — ■ In the leaves of a corn plant is a 

 certain set of cells, known as buUiform cells. These are 

 located near the upper surface between the ribs, or veins. 

 (See Fig. 14.) When moisture is abundant, these cells 

 absorb water until they are turgid. The leaf is then 

 spread out flat and is more or less rigid and brittle. How- 



