Friday, April 6, 2012

Leaf it to me, Buddy!

While we celebrate fall colors in Wisconsin for their variety and vibrancy, spring colors will also impress the attentive observer. After the snow melts and we enter another round of “stick season,” the drab woods can be a little discouraging. Grass and weeds green up first, since many of them are immigrants from the Russian steppe. To survive in this cold grassland, the plants have adapted to breaking dormancy at much lower temperatures than our locals.

Many leafy things that are green now – Tartarian honeysuckle, common speedwell, lilacs, and dandelions – are not native here. The few evergreen natives, like wintergreen, partridge berry, clubmoss, and pipsissewa, have thick, waxy leaves to protect from frost and desiccation.

If we are patient, color rises slowly in the trees, and soon the forests are washed with the soft greens and pinks of bursting buds and fresh new leaves. Those buds formed months ago, during the steamy days of late summer. At that time, the plant organized the basic cells for shoots, leaves, and flowers, and encased them in protective scales or thick fur.

All winter, tiny and important, they waited for the right cue. Some did not survive. Grouse, purple finches, deer, squirrels, moose, rabbits, and hares all know what a fine winter food source those little packets are. Bright red basswood buds are sweet enough for me to nibble, too.

But what is the right cue? Naturalists have pondered this for many years, and struggle to design experiments that can control all the variables and provide answers that we can generalize across species and locations. The best explanation is that bud-break is determined by a complex interplay of factors involving genetics, day length, cold exposure, and warmth.

Once bud-break happens, there are still more mysteries to ponder. Ever since I can remember, miniature spring leaves have fascinated me. Oak leaves in particular start out wonderfully red and fuzzy, with all their little lobes and wrinkly veins. The rich color is a result of anthocyanin, the same pigment that protects leaves in the fall. Before a leaf has filled with chlorophyll, excess sunlight can be damaging. Anthocyanin acts as a sunscreen and anti-oxidant. The fuzz protects tend young leaves from frost the same way a wool sweater keeps you warm – by trapping warmer air next to the surface.

Leaf growth is another natural mystery. A leaf's size is determined by a combination of cell number, cell size, and intercellular space. Leaf cells within the bud are pre-programed to grow with a certain pattern, and emerging leaves use the plant’s built-in orientation system to determine their axis of growth.

Just like scientists have developed a computer model to simulate birds and fish moving in flocks or schools, they have created a computer model that uses simple rules of leaf growth to grow an accurate “virtual” leaf.

Cells at the leaf margins and on the leaf’s surface layer are especially important in determining leaf and petal size. They are genetically programmed to secrete growth hormones that encourage leaf cells to divide. Once the hormone is diluted to a certain point, growth stops. Animals use this same principal of dilution (although with different hormones) to determine size (like on the wings of a fly, for example). This uneven cell growth results in leaves and flowers with characteristic sizes and shapes that we recognize.

Once leaves mature, they begin to photosynthesize. Using energy transferred from photons of sunlight to chlorophyll molecules and into a complex photosystem. Then plants can break apart molecules of carbon dioxide and water and re-combine them into sugars. From simple sugars, they make carbohydrates and cellulose, and with those building blocks, they begin the process of forming new buds for next spring.

How quietly, and not with any assignment from us, or even a small hint of understanding, everything that needs to be done is done. -- Mary Oliver, “Luna”

For over 44 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 13470 County Highway M. The new exhibit, STAR POWER: Energy from the Sun, opens in May 2012. Find us on the web at to learn more about our exhibits and programs. Also discover us on Facebook, or at our blogspot,

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