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A torus-margo pit observed with scanning electron microscopy.

In the center of the pit is the torus; water flows through the large pores in the margo. Notice how pore size changes from relatively large pores closer to the torus to smaller pores at the periphery of the margo. Most of the water flows through the large pores. Efficient inter-tracheid pits are an important reason why conifer trees can be so tall; see this paper.

Photo: Amanda Schoonmaker