ANATOMY OF THE CELL. 39 
14. Organic Acids. 
Oxalic acid is found, like many other acids, free in the cell 
sap, but is most frequent in combination with calcium. As 
calcium oxalate, it is found in the various kinds of crystals 
occurring both in walls and in contents. Calcium oxalate is 
considered by most authorities as a waste product, not afterward 
used in the formation of the parts of the plant. Malic, acetic, 
citric, and formic acids, also others of less frequent occurrence 
than these, are found either free in cell sap or in combination 
with calcium or potassium. These acids occur mostly in fruits, 
but are also plentiful in other parts of plants. 
There is another group of astringent substances which also 
possess a faint acid character. The most important of these are 
tannin, or tannic acid, gallic acid, and the astringent principle 
in Cinchona, Catechu, and Kino. These substances are usually 
found dissolved in the cell sap. Tannin is also found in little 
globules of solution enveloped by a delicate film of albuminous 
matter. It is sometimes placed in the next category, namely, 
among the glucosides. Nothing is positively known in reference 
to its function, and there are various theories regarding its 
origin. 
15. Glucosides. 
This name has been given to certain substances which by 
the action of unorganized ferments are broken up into glucose 
or some allied sugar and some other body capable of farther de- 
composition. The more common glucosides are Salicin, Phlor- 
idzin, and Coniferin. 
16. Alkaloids. 
Among the nitrogenous waste products of plants ammonia 
compounds’ are now considered the most important. Those 
which are not volatile at ordinary temperatures are called al- 
