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It is evident that water plays a large part in the economy of the tree. It occurs in wood in three different ways: In the sap which fills or partly fills the cavities of the wood cells, in the cell walls which it saturates, and in the live protoplasm, of which it constitutes 90 per cent. The younger the wood, the more water it contains, hence the sap-wood contains much more than the heart-wood, at times even twice as much.

In fresh sap-wood, 60 per cent. of the water is in the cell cavities, 35 per cent. in the cell walls, and only 5 per cent. in the protoplasm. There is so much water in green wood that a sappy pole will soon sink when set afloat. The reason why there is much less water in heart-wood is because its cells are dead and inactive, and hence without sap and without protoplasm. There is only what saturates the cell walls. Even so, there is considerable water in heart-wood.²

The lighter kinds have the most water in the sap-wood, thus sycamore has more than hickory.

Curiously enough, a tree contains about as much water in winter as in summer. The water is held there, it is supposed, by capillary attraction, since the cells are inactive, so that at all times the water in wood keeps the cell walls distended.