CRYPTOGAMS 



263 



between sterilized and unsterilized soil in supplying nourish- 

 ment to plants? 



Note. If a sample of soil should contain harmful bacteria, 

 sterilization would kill them and the soil would be better for the 

 plant than unsterilized soil, for then the plant could absorb such 

 mineral foods as might be present without interference from bac- 

 terial foes. 



Leguminous plants, such as clover, alfalfa, beans, peas, etc., 

 usually have swellings upon their roots which are the home of bac- 

 teria. These organisms perform valuable work by absorbing 

 nitrogen from the air, which they build up into nitrites and nitrates. 

 In this way, plant foods are formed. The process is known as 

 fixation of nitrogen, and such bacteria are called nitrifying bacteria. 



Cells and Protoplasm 



If the student refers to the various demonstrations with 

 the microscope (Exs. 54, 91, 108, 109, 146, 147, 198, and 215), 

 he will find that on close inspection all these plant structures 



FIG. 115. Spirogyra. 



are built up of very small units called cells. In seeds of 

 the horse bean ( Vicia faba) the cells can be plainly seen with 

 the naked eye or with a simple magnifier. 



If sketches of all the different forms of cells are made and 

 compared, they will be found to have three points of simi- 

 larity; namely, a very delicate sac or membrane, the cell 

 watt, containing a clear transparent fluid, the protoplasm, 

 and suspended somewhere within it, a tiny mass of slightly 

 denser material, the nucleus. 



