We explored how water moves through trees and explored all of the spring growth on our neighborhood trees!

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Christina Smith, a graduate student in Plant Biology at the University of Minnesota, helps a market-goer examine tree stems under a microscope.

Like all living things, plants need water. They use it for photosynthesis (the process of using energy from light to make sugars) and to keep their tissues hydrated. Plants, however, don’t have any organs that pump water throughout the plant. We examined sections of trees for evidence of how plants move water.

We put a section of wood under the microscope to look for xylem, the cells that act as water pipes.

Plants move water throughout the plant in a series of pipe-like specialized cells, or xylem. Xylem tubes, or vessels, are connected to one another from the roots, through the stems, and into the leaves. We learned about how water moves through the xylem because of the Soil-Plant-Air Continuum.

We looked at the evidence of the Soil-Plant-Air Continuum in real plants! We could see xylem cells and stomata under the microscope.

For most plants, water in the soil is the source of water and roots absorb this water so that it can fill the xylem. Water moves out of the plant through the leaves. The leaves have small pores, stomata, which open to take on carbon dioxide, CO2, which the plant uses for photosynthesis. The leaves are like small bags of water, while the air outside is much more dry (not 100% humidity). As would happen if a glass of water is left outside on a warm day, water is lost to evaporation. So when the stomata on a plant’s leaves are open, CO2 comes in, while water evaporates out.

We looked at the underside of leaves of purple spiderwort (Tradescantia pallida) to find the leaf pores, or stomata. Even at only 3.5x magnification, the green guard cells around the stomata stand out against the purple leaf.


This loss of water exerts a negative pressure akin to the sucking of liquid through a straw. When the plant loses water in its leaves, it pulls more water from the xylem (in the leaf veins) and that pulls more water from the xylem in the stem, which pulls more water from the xylem in the roots, which can then absorb more water from the soil.

We had a demonstration of the Soil-Plant-Air Continuum: we tried our hand at being the atmosphere and pulling water through leaf stomata with a paper straw. Negative pressure can really move water.

We also explored how this method of water movement and the seasons in Minnesota can cause tree rings. Many of us are familiar with annual rings on trees, but we explored why many trees form rings here, while they might not in other areas (like the tropics).

Many sections of trees: we could see rings on the trees from around here because the trees only grow between spring and summer and because most of these trees develop different sizes of xylem between spring and later summer. (Thanks to Cindy Buschena at UMn Forest Resources for the great wood sections!)

Trees form xylem throughout their growing season, from spring to early fall. They also have access to different amounts of water throughout their growing season. In the spring, the soil is wetter with snow melt and spring rains, while by August, it’s the air is hotter and there isn’t as much rain. Because water moves through the plant as it’s sucked out by the atmosphere, plants without much access to water can form bubbles in xylem, which stops water flow (this is somewhat similar to form bubbles in a straw when near the end of a drink). Plants can avoid that by forming smaller (thinner diameter) xylem in dry conditions. Because the trees in Minnesota form xylem of different sizes each growing season (wide in spring and thin in late summer) we can see annual rings.

We couldn’t see all the xylem cells on this red pine from Itasca, but we could count that it was about 120 years old! We could also tell that it survived a fire: the rings kept forming around a big fire scar on the lower side of the trunk.

Since we knew that trees are moving water and adjusting their xylem cells sizes to the environment, we could understand that trees are really dynamic, living organisms. We examined some local tree twigs to find flowers and cones and  to see leaves and young wood up close.

Neighborhood twigs for examination. Some of the plants wouldn’t grow here naturally, like the gingko in the front.

We didn’t have many twigs with only buds to examine because our neighborhood trees leafed out a bit earlier this year than last, possibly because we had a very warm spring. Buds contain tissue that forms or expands into the first leaves and flowers of spring. They overwinter on the trees. There are several factors that cause the formation: the trees here in Minnesota form buds in response to a shortening day length and longer night length (and sometimes cooler temperatures) which indicates the onset of later autumn and winter. Trees have a method of tracking cold days during the winter and once they have experienced enough cold days, they will then begin to expand buds when temperatures warm. This can be a problem if temperatures warm too early and new leaves experience a frost. Plant buds also sense lengthening days and can leaf-out in response to the longer days of spring.

Thanks to University of Minnesota plant biology graduate students (L to R) Allison Haaning, Leland Werden, Christina Smith, and Diana Trujillo (not pictured) for discussing dynamic trees!

We explored how plants move water and looked for evidence in leaves and stems. We also learned that tree rings tell us more than a tree’s age: they also show the transition from wet to dry environmental conditions. We then check out what trees in our neighborhood were doing in response to a warm spring.

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