CRYSTALS 15 



but in some plants cells are found which contain so-called 

 "ethereal oils," which are not true fats. 



26. Crystals. In many plants may be found cells 

 containing crystals. These may be cubical, prismatic, 

 regular or irregular polyhedrons, needles, compound 

 crystals, etc. Sometimes the cells containing them are 

 unchanged but often they are enlarged or of special 

 shape. This is especially the case with the needle- 

 shaped crystals which are called raphids 

 and occur in large bundles in the cen- 

 tral vacuole of rather large, thin-walled 

 cells. The crystals seem to be formed 

 by the cytoplasm, in which they occa- 

 sionally lie, or more frequently in special 

 small vacuoles in the latter. Eventu- pound,' and needTe- 



,1,1 ~ 1. , ■,! shaped crystals. 



ally they are lound in most cases m the 



central vacuole in which some of them may have had 



their origin. 



27. Crystals in most plants are composed of calcium 

 oxalate. In some plants calcium carbonate crystals 

 occur, while crystals of still different composition are 

 occasionally found. The purpose of crystals is not clear 

 in all cases but in many cases they are probably the 

 product of the combination of waste substances set free 

 in the course of some of the important chemical pro- 

 cesses of which the cell is constantly the seat. 



Laboratory Studies, (a) Make a thin section of a potato 

 tuber. Mount in water. Note the large, thin-walled cells 

 packed with numerous ovoid, concentrically marked starch 

 grains. Treat with iodine solution. The starch grains become 

 blue or purple. In very young tubers, where the starch grains 

 are not so large nor so numerous, they may be seen to be 

 enclosed in leucoplasts. 



(6) Study the different types of starch grains in corn, wheat, 

 rice, oats, etc. 



