Missy Holbrook’s sunlit office is dominated by a large Boston fern, bursting with life, its exploratory tendrils shooting far up the walls and drooping, beardlike, to the floor. Nearby, a sweet potato vine twines gently around the vertical slats of the window blinds, squeezing them in its progress toward the ceiling.
Many people think of plants as a blurry green backdrop to the “main event” of people and animals going about their business. But in Holbrook’s worldview, plants fill the foreground, seething with vital internal processes. Plants organize the Earth, creating habitats for animals; they lock carbon in their bodies, mitigating the greenhouse effect; and they provide interfaces for microbial life, where a lot of chemical transformation takes place.
Plants are also the major means by which water moves back into the atmosphere. As a professor in Harvard’s Organismic and Evolutionary Biology Department, and as one of the world’s leading plant physiologists, Holbrook takes special interest in this fact.
Every day an oak tree moves hundreds of gallons of water up from the soil and out, in evaporated form, through its leaves. “Mechanically, it’s a pretty substantial feat,” says Holbrook. “They do it very quietly, with no moving parts, but you’d be hard-pressed yourself to get that much water out of the soil and into the atmosphere.”
Learning how plants move water has implications for agricultural productivity, but also for problems in engineering. For several years, Holbrook and her colleagues have studied the long-distance transport of water, as well as sugar, in plants. Some might think of the parallel tubes that transport water and sugar – the xylem and phloem, respectively – as a sort of king’s highway along which the water and sugar travel on mules to their destination. Holbrook conceives of that “road” as highly interactive with the traveling fluids – something of a “smart” road – and including not just mules but clever messengers – ions disguised as royal envoys, and pressure signals as child spies.
In a way, says Holbrook, the transport system is akin to the “brain” of the plant, an admittedly inexact analogy given that plants are fundamentally decentralized, with no single unit that processes information the way the brain does in the human body.
Plants are brilliant, in this sense: They’ve found mechanical and chemical solutions to problems that so far can’t be reproduced in a lab. Sunlight heats water molecules in a plant, for instance, pulling them in a fragile water column toward evaporative surfaces on leaves. As molecules fly off through evaporation, the chain of sticky water molecules remains unbroken; in part, capillary action pulls other water molecules into place to help keep the water column intact.
Holbrook and her colleagues have found that plants can reconnect breaking water columns partly because fluctuating concentrations of ions in the xylem cause its pathway to widen or narrow, which helps to maintain sufficient pressure that the water chain can remain unbroken.
Conversely, sugar transport in the phloem is threatened by too much pressure. If too much sugar is pushed into the phloem, its viscosity will prevent it from moving. Holbrook, her former Ph.D. student Mathew Thompson, and research associate Maciej Zwieniecki have been studying the way phloem tissues use ions such as potassium to keep the pressure high enough – and the viscosity simultaneously low enough – that the flow can continue.
The xylem and phloem, which exist side by side, interact in their use of ions, Holbrook has found. Holbrook and her collaborators are exploring the use of magnetic resonance imaging (photographing the interiors of plants) and the use of “hot isotopes” (very high-energy, short-lived isotopes) to help track the process of sugar through the system.
‘Tell me why the ivy twines’
Maybe it’s telling – Holbrook likes “oddball plants.” She likes strangler figs, which develop in the branches of a host tree and then send down roots that ultimately strangle the host tree. She loves black gum trees, which grow asymmetrically, with their scaly bark and their arms of flame when the leaves senesce in the autumn. She likes difficult environments, like mangrove swamps, where the flooded soil makes oxygen intake hard for plants, and where the water swings from warm to cold, from briny to fresh, requiring continual adaptation.
But Holbrook also likes common plants, as a glance around her office suggests. And she finds sources of wonder (and research projects) in the most ordinary surroundings. “Every day as I walk to school, I check out all the different bushes and plants,” she says. She sees oak leaves turning scarlet. Why do oak leaves senesce later than maples? She sees the leaves of the Harvard rhododendrons curling. What does this say about how plants cope with freezing? How do flowers keep from wilting? Why do maple trees make sap? “It has nothing to do with making pancake syrup,” she says.
“There’s always plenty to see. As I tell my botany class, the thing about being a botanist is, you will never be bored.”
‘The forest revealed itself’
Holbrook grew up in the suburbs of Washington, D.C. After her sophomore year at Harvard, she spent a year studying bees on an island in the Panama Canal – Barro Colorado Island, a research station for the Smithsonian Institution set in the midst of a tropical forest.
“It was fabulous, it was transforming,” she said. “I just read, talked, and walked, and counted bees, and I loved it.”
“I spent a lot of time in the forest, quietly. From dawn until way after dark.” Over time the forest revealed itself. “When you first get there, everything looks like walls of green. You can’t see one plant from another. But the more you learn, the more you see. You’ll never see everything, because the diversity’s too great.”
After graduating from Harvard in 1983, Holbrook received her master’s degree in botany from the University of Florida in 1989. After earning her Ph.D. from Stanford in 1995, she became an assistant professor of biology at Harvard, an associate professor in 1999, and received tenure in 2001. She has received teaching awards at both Stanford and Harvard. Her research is branching out to the roots and leaves: the mechanics of how roots interact with the soil, and the hydraulics of leaf production, flowering, and shedding. She has projects, with her collaborators, that take her from Chile to Costa Rica to Brazil to Australia, to Venezuela. Never bored.
“The natural world is so beautiful, and so interesting. Being outside makes me happy,” she says. “I think it is that simple.”