Figure 1.

(i) A schematic of our low field NMR system used to measure the water content in a tree shows (a) the 41 cm diameter Helmholtz coils which produce ${\overrightarrow{B}}_0$; (b) copper shielding, with one panel removed for clarity, used to block rf noise; and (c) the solenoid, wrapped around a former on the tree's trunk, which both produces ${\overrightarrow{B}}_1$ and detects the resulting NMR signal. (ii) A representative neutron image of a branch showing (A) the stem; (B) the NMR solenoid; (C) the cup holding the heavy water; and the regions of interest as highlighted by the white boxed. (iii) Subtracting subsequent neutron images from the first creates difference images. Such images clearly show the uptake of the heavy water through the branch, even revealing a section of non-conductive heartwood running down the center of this branch. Difference images also help reveal any physical movement of the system, and provide a direct measurement of the amount of water taken up by the plant (D).