210 THE SECOND BOOK OF BOTANY. 



circles indicate so many layers superposed around a 

 little kernel indicated by the central point. 



In looking, with the microscope, at sections of 

 leaves, you will see gelatinous flocks of green matter 

 swimming in the colorless liquid of the cells, or de- 

 posited on the cell-walls and grains of starch. This 

 substance, to which vegetation owes its green color, 

 is called chlorophyll. The yellow coloring-matter of 

 plants is like chlorophyll in every respect, except its 

 color, but the red, violet, and blue coloring-matters 

 are always liquid. 



The colorless sap of plants, which fills the cells 

 and vessels, holds in solution all the materials of cell- 

 growth, and of the substances contained in cells. Su- 

 gar, dextrine, and gum, dissolved in water, are found 

 in the cells, the intercellular spaces, and lacunes, but, 

 being held in solution, they cannot be detected by 

 the microscope. The intercellular spaces, also, fre- 

 quently contain air. The fixed oils found in seeds and 

 fruits, and other parts of the plant, form isolated glo- 

 bules, that, by pressure, flow together into large glo- 

 bules. Essential oils, turpentine, and caoutchouc, are 

 usually accumulated in intercellular cavities, or given 

 oif at the surface when the plant is wounded. 



Yarious mineral matters are also taken up by the 

 roots from the soil, dissolved in water, and deposited 

 in the structure of the plant. They occur sometimes 

 in the crystalline form in cells. Indeed, it is said 

 that almost every herbaceous plant contains them in 

 more or less abundance. Fig. 381 represents cells of 

 rhubarb, from one of which needle-shaped crystals, 

 called raphides, are being ejected. Cells of this kind 

 in the stalks of rhubarb, when moistened with water, 



