Plant Organs Stems Part 1

Roots – STEMS – Leaves – Flowers Fruits • Stems provide support for plants, contain a system of tubes (vascular tissues) for moving liquids and nutrients throughout the plant, and are points of attachment for leaves and flowers. The twisted stem of this tree is still able to support its canopy. Photographed at Gooseberry Falls State Park, MN. Herbaceous vs. Woody Stems • Herbaceous stems are soft, flexible, and green; made up mostly of primary tissues, covered by epidermis, and do not show great increases in diameter; most are annuals, meaning they live only one year. • Woody stems are rigid, covered in bark, possess secondary tissues (e.g., wood), and increase in diameter over many years of growth. TOP: Woody stems of Sitka Spruce can grow many feet in diameter and the trees can exceed 250 feet in height. BOTTOM: Herbaceous stems like those of Common Milkweed are green and flexible. Woody and herbaceous stems, as shown here, are also described as aerial stems because they grow up out of the soil into the air. Herbaceous Monocot vs. Herbaceous Dicot Stems • Monocot stems have scattered bundles of vascular tissue (xylem and phloem)—the bundles look like little “monkey faces” under the microscope. • Dicot stems have bundles of vascular tissue arranged in a ring separating the outer tissues from the inner pith (parenchyma at the center of the stem). ABOVE: A cross-section through a corn stem, left, and a single vascular bundle (“monkey face” from a monocot stem. BELOW: A crosssection through an herbaceous dicot stem (Coleus), showing the ring of vascular bundles surrounding the pith at the center. Woody Dicot Stems • Dicots that develop woody stems demonstrate secondary growth—as they grow, they add on to the original (primary) vascular tissues within the plant. • Woody dicots have two important outcomes from secondary growth, namely wood and cork. • Wood and cork develop from actively dividing cells in the vascular cambium and cork cambium, both of which are part of the lateral meristem of a woody plant. Let’s take a closer look at a woody dicot stem; in this case, it’s a 4-year-old tulip-tree (Liriodendron)… Cork cambium Secondary Phloem Cork Annual growth rings in secondary xylem Primary phloem Cortex Year 4 Year 3 Primary xylem Vascular cambium Year 2 Xylem rays Pith Year 1 Woody Stems • Woody stem features: • Heartwood—after several years, the secondary xylem toward the center of the stem (trunk) is somewhat crushed and no longer carries any materials up through the stem. • Sapwood – not surprisingly, sapwood is xylem that is still capable of moving materials up through the stem. • Bark includes the secondary phloem (in some books) and layers of cork (called periderm here). Challenge question: which layer of this woody stem is being tapped to extract the liquid that will be boiled down to make syrup? Growth rings in woody plants Growth rings are the alternating light and dark bands in the secondary xylem of a woody stem. The width of each ring can reveal information about environmental conditions that occurred during each year of a tree’s life. In years with lots of rain, the rings will be wider because the rain boosts growth; in dry years, the tree can’t grow as much due to lack of water, so the rings will be narrower. Seasonally, spring tends to be wetter, while fall is dryer, as well (as shown in the inset), so within a single year, one can analyze conditions in which the tree grew. If you counted about 24 annual rings, you’re correct! Dendochronology • Dendrochronology is the scientific study of tree growth rings. • Dendrochronologists are working to provide information about past climate, weather, and natural disasters. Their work even helps historians explain some of the effects of weather events or climate change on human settlements. • Part of your lab this week is to practice the science of dendrochronology on a sample slice of a tree, so make sure you study these slides carefully before you attempt the lab. Here is a massive slab of wood cut from a very old Douglas Fir tree, on display at the Olympic National Park Visitor Center in Port Angeles, Washington. The tree itself was more than 650 years old when it was cut down! Note the white arrows on the slab. Each one identifies an important point in the tree’s history or human history. You will do something similar with a much smaller specimen during this week’s lab! Photo: L. Wentz, 2019