This, when pure, and ritually prepared, was supposed to wash away all sins, and to qualify the priest to approach the altar of the gods with more acceptable prayers: washing with water being a type natural enough of inward cleansing and purity of mind.
They also worshipped fountains and lakes and rivers.
Oaks were regarded by this sect with a particular veneration, as, by their greatness, their shade, their stability, and duration, not ill representing the perfections of the Deity.

From the great reverence in which they held this tree, it is thought their name of Druids is derived: the word Deru, in the Celtic language, signifying an oak.

But their reverence was not wholly confined to this tree.

All forests were held sacred; and many particular plants were respected, as endued with a particular holiness.

No plant was more revered than the mistletoe, especially if it grew on the oak,--not only because it is rarely found upon that tree, but because the oak was among the Druids peculiarly sacred.