Water flow through plants is critical to our food supply: without proper water flow, plants cannot carry out photosynthesis, or grow or reliably produce flowers, fruit or seeds. Water flows within specialized structures from the roots, through the stem to the leaves, where its evaporation is regulated by microscopic pores called stomata. All the water-conducting structures are produced by tightly controlled developmental sequences: cells must divide to produce the necessary cell types in the correct place at the right time. But many details of how these complex developmental processes are controlled remain unclear.