In these the variety of tannic acid is not exactly the same, but although there are slight chemical differences, they all possess the power of tanning raw hides and of preserving albuminous tissues.
The sieve tubes contain a thin lining layer of protoplasm on their walls, but no nuclei, and the cell sap contains albuminous substances which are coagulable by heat.
The element also occurs in the animal and vegetable kingdoms. It is present in hair and wool, and in albuminous bodies; and is also a constituent of certain vegetable oils, such as the oils of garlic and mustard.
The vegetable kingdom is the original source of albuminous substances, the albumins being found in greatest quantity in the seed.
Albuminoids is the anatomical name given to albuminous substances forming the connective tissues.