40 ELEMENTS OF PLANT ANATOMY. 
kaloids ; they have an alkaline reaction and are poisons. Their 
presence has been detected in a large number of plants, and 
they are especially deposited in those parts which become de- 
tached, such as bark, fruits, and seeds. There are two kinds 
of alkaloids, those which do and those which do not contain 
oxygen. 
17. Crystals and Cell-sap. 
Although plant cells contain a large number of crystallizable 
substances, the crystals contained in them are nearly all com- 
FG. 20. 
Crystals of calcium oxalate. a-c from leaf of Tradescantia discolor. d from cactus stem. 
e and f from leaf of Iris. a-e x 300. d-f x 300. —( Wiesner.) 
pounds of calcium with some acid. Crystals of calcium oxalate 
are by far the most common. They are found not only in cell 
sap but also in the walls, and are very widely distributed, nearly 
all plants containing them in some form. Compounds of calcium 
with phosphoric and carbonic acids have been found in a few 
instances. 
Cell-sap is the name given to the watery fluid which consti- 
tutes the vacuoles. It is customary to consider it in connection 
with all the substances dissolved in it, which are mostly the or- 
ganic compounds before described. OQwing to the presence of 
the numerous acids it generally has an acid reaction. 
