Previous work by the author on the water contents of Canadian trees is reviewed and followed by a brief discussion of questions yet to be answered.In Betula alba v. papyrifera, in at least the young parts of B. alba v. pendula laciniata, in B. populifolia, and in several sizes of Populus tremuloides, there is a marked seasonal rhythm in water content. The maximum is at leaf opening, the minimum at leaf fall. In poplar but not in birch there is a very high water content in December. During winter a considerable loss of water may occur. A winter loss is shown also by the wood of hemlock and larch and by twigs and leaves of white pine and hemlock. Losses from leaves are surprisingly small.The behaviour of B. populifolia has been studied for more than three years, and differences have been correlated with observations on weather conditions. Experimental work on movement of water in this species during winter is inconclusive. This work continues.