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MODIFIED STEMS

STOLONS

Stolons and tubers on Nut-GrassStolons are slender stem-branches running horizontally away from the main plant, either above or below ground. In the picture at the right you see two plants of Nut Grass, Cyperus esculentus. It's not a grass at all, but rather a member of the Sedge Family, but that's a different story.  Anyway, I just dug these two connected Nut Grasses from my garden and they tell a story. 

The larger plant at the left in the picture (it's about 8 inches tall) is the "mother plant." It has issued a stolon that has grown about six inches through the soil, then the stolon budded and from the new bud arose the smaller, younger plant at the right in the picture. Note that some of the stolons end in roundish, potato-like tubers. There's more about tubers below.

The stolons you see in the picture arose from the same general area on the plant as the roots but, again, they aren't roots. They're stems because they sprout at their nodes, and real roots don't have nodes. (Nodes are explained at the bottom of our Stems Page.)

Bill Stringer, a forage agronomist at Clemson University in the US, says that in agronomy stolons are regarded as aboveground runners only, with rhizomes being their underground counterparts. Other sources in basic botany, however, say that  stolons can grow either above or below the ground's surface.

One reason that Nut Grass and some other weeds are so hard to get rid of in my garden is that they are very stoloniferous -- when you pull up the plants, you're bound to leave behind at least one stolon section, from which a whole new plant can arise.  By the way, those tubers on the Nut Grass are only about the size of large peas. Nonetheless, to our ancestors such tubers often meant the difference between survival or starvation.  The weedy Nut Grasses I try so hard to keep from my garden today may be the descendents of Nut Grasses that once saved my ancestors' lives!

TUBERS

potato with sproutsTubers such as  those shown at the tip of some Nut Grass stolons above, as well as the ordinary potato shown at the right, are often thought of as roots. However, as we've just said, roots don't have buds, and that's exactly what you see sprouting on the potato, arising from the potato's "eyes." Tubers are actually swollen portions of underground stems (stolons) and, as we've seen, stems have nodes, and buds arise at nodes.

One reason it's hard to think of the potato with its sprouting eyes as an underground stem is that no nodes are obvious. If you were a scientist able to watch the potato's cells divide and grow from the very beginning shoot developing from eye of potatoyou'd see that in the very early stages of development the potato had recognizable nodes, and then you could watch the nodes develop slowly into the potato's eyes, and the eyes would have buds associated with them, just like a normal tree-branch node.

At the left you see a close-up of two sprouting buds in one of the above potato's eyes. turnip, a kind of storage rootDo you see the future leaves and stems at the top of the two sprouts? At the bottom of the egg-shaped sprouts you can see pale bumps that will develop into roots. Each of these sprouts has the potential for being an entire potato plant with its own potatoes.

By the way, you may have never even seen potatoes sprouting like the one in the picture. Usually potatoes sold in stores are sprayed with chemicals to keep them from sprouting. That's one more reason to eat organically grown food when you can.

At the right you see a turnip from my garden. The large, purple part is a tuber producing roots only on the slender tap-root beneath it. The turnip-plant's stem is shortened into a kind of "neck" atop the tuber. When the plant matures more, a regular stem bearing flowers will arise from the "neck."

RHIZOMES

Rhizome of Johnson GrassAt first glance rhizomes are like underground stolons, but there's an important difference between them: Each stolon is just one of what may be several stems radiating from the plant's center. Rhizomes, in contrast, are the main stem. If a tree grew with its trunk horizontal below the ground, with its side branches emerging aboveground, the buried trunk would be a rhizome. The thick, fleshy "roots" of irises, cannas, and water lilies are actually rhizomes. So are the whitish, thumb-thick items at the right. What you see there are the succulent rhizomes of Johnson Grass, just dug from my garden. The horizontal part was growing about an inch below the ground's surface. In the picture you can spot the nodes in the horizontal section because the nodes are dark brown, while the internodes are mostly whitish.

CORMS

If you take a regular, aboveground, single, straight stem with its various nodes, and, keeping it standing vertically, squeeze it downward until it becomes wider than tall, and bury it underground, you'll have a corm. Gladiolus cormCorms, then, are unlike stolons and rhizomes because they usually grow vertically, instead of lying horizontally. They're unlike tubers in that  tubers are typically attached to the main plant by a slender rootlike part of the stem, a sort of umbilical cord, while corms constitute the below-ground "heart" of the plant, the part from which aboveground stems and leaves directly sprout. In the corm shown here, notice the horizontal band running across its middle. That's a node just like the nodes that are so conspicuous on the bamboo stem at the bottom of our Stem Page. Notice the roots emerging from the base of the corm. Gladiolus, crocus, and tuberous begonias all arise from corms.

BULBS

Bulbs can be considered to be very short stems encased in thickened, fleshy bulb scales (which are modified leaves). As the drawing below shows, the two basic bulb types are layered and scaly:

layered bulb and scaly bulb

  • LAYERED BULBS are composed of a series of fleshy scales that onion bulb cross section, with garlic bulb showing clovesform concentric rings when the bulb is cut in cross-section. In the picture at the right, both the onion bulb on the left and the garlic bulb on the right are layered bulbs. Well, the onion bulb is easy to recognize as a layered bulb but the garlic bulb is tricky because it looks like a scaly bulb. The difference between a garlic bulb and a scaly bulb is explained in the following section.
  • lily bulb, a kind of scaly bulbSCALY BULBS, such as the lily bulb at the right, have fleshy bulb scales, which are modified leaves loosely clustered around the stem base. How are scaly bulbs different from the garlic bulb shown above? Each of the garlic bulb's cloves (the smaller bulb sections in the picture) is itself a small layered bulb. Visualize garlic as having a very short, flattish stem, same as the onion in the drawing above, and imagine buds forming in the leaf axils of that squashed stem, exactly as on a normal, much longer stem. Those buds then enlarge to form the garlic's cloves, which obliterate the leaf petioles as they grow larger. Therefore, the garlic's cloves are actual bulbs developed from buds. In contrast, each section or "scale" of a scaly bulb is a modified thick and fleshy leaf. The scales serve as sites of food accumulation. In the spring when the lily stem shoots up from the center of the scale cluster, it will draw its food from the scales.

WATER-STORING STEMS

cactus with a water-storing stem, image by Karen Wise of MississippiThese stems, you might guess, are stems specializing in storing water for the plant's use between rains. Instead of being woody, like tree stems, usually they are fairly soft and uncommonly thick, or "bloated-looking." The most famous such stems are those of the cacti, one of which is shown at the right. Another common potted plant with water-storing stems is the Jade Plant. Backyard weeds with water-storing stems include spurge, purslane, and milkweed.