ANATOMY AND MORPHOLOGY OF THE FLOWER 233 



inexpansible to pressure from within, which can be done by 

 winding it tightly and carefully by several turns of tire- 

 tape (used for repairing bicycle tires). Enough water should 

 then be put into the tube to bring it up to the zero mark, 

 and the stop-cock (a perfectly air-tight one) should then 

 be closed. The water forced out from the stump will then 

 compress the air column, and the exact pressure exerted may 

 be calculated by Mariotte's law, that pressure is inversely 

 proportional to the volume of the gas. Thus, suppose the air 

 column is compressed to three-fourths of its former length ; 

 this means a pressure upon it of four-thirds. But it had one 

 atmosphere, that is, three-thirds, upon it at the start ; hence 

 the additional pressure exerted by the water will be one-third 

 of an atmosphere, or about five pounds to the square inch. 

 This implies that the readings shall be taken always at the 

 same temperature, which is not difficult to manage, and it 

 neglects a slight error due to the water vapor in the tube, 

 but the latter is at the most very small. The ingenious teacher 

 can make a tube without the stop-cock, perhaps even a closed 

 test-tube, do. The plant is, of course, to be watered regularly, 

 but not too much, or the roots will soon die of suffocation. 

 Another very instructive experiment upon roots is one to 

 show their hydrotropism, a most important irritable property ; 

 methods of demonstrating it are given in all physiological 

 works. 



XIV. The Anatomy and Morphology of the 



Flower 



52. What is the structure of the essential parts of the 



flower - - pollen-grain and ovule ? 

 Answer from a study of the material supplied, 



