—— ee Se ee a? 
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 tt me ee 
small vacuoles in the latter. Eventu- pound, and needle- 
ally they are found in most cases in the aS See 
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. 
