124 AGRICULTURAL BOTANY 



the roots spread over the surface, slight grooves will be etched through 

 the polished surface of the marble. This is done by the acid that the 

 root hairs give out. 



FIG. 51. Marble corroded by Bean Roots (Minnesota Experiment Station). 



Osmosis. --To understand just how the root hairs 

 absorb moisture and plant food held in solution, it 

 will be necessary to examine more closely the structure 

 of the hairs. As stated above, each root hair is a 

 cell filled with protoplasm and cell sap. The former 

 usually lines the inner surface of the cell wall and has 

 one portion more dense, called the nucleus. The nucleus 

 is supposed to be the center of the life of the cell, but 

 its use is really little understood. 



The cell wall has no pores, or openings, that can be 

 seen with the highest power microscope made, yet 

 water can pass through the wall with ease. It has 

 been found that some plants having a large root sur- 

 face have absorbed many times their own weight of 

 water in twenty-four hours through the thin walls 



