ARTHUR C. GIBSON, MEMBG Director
UCLA is known for its track stars, and has produced some especially good runners. Think about Evelyn Ashford, Gail Devers, Jackie Joyner-Kersey, Florence Griffith-Joyner, back further Ato Boldon, Kevin Young, Danny Everett, and John Smith. Former Chancellor Charles Young, the leader at UCLA for more than a quarter century, jogged around campus in the early hours. Campus is, by some standards, a great place to jog, if you do not slip on fig fruits or trip over their exposed buttress roots. But this article is about runners of plants, or what are termed stolons. A stolon is a specialized type of horizontal, above-ground shoot that enables a plant to clone itself. You would never receive skin abrasion by tripping over a lowly stolon.
A stolon is a stem with long internodes between the nodes, and the stem diameter tends to be much thinner than an otherwise "normal" stem on the plant. They arise from axillary buds at the plant center and grow rapidly and horizontal to the substrate. Where a node contacts moist soil, adventitious roots emerge and anchor the structure, and from the bud at that node often there arises the new plantlet.
The most common textbook example of a stolon is the strawberry (Fragaria, Rosaceae), in which the mother plant forms plantlets on stolons during spring growth. In the case of the strawberry plant, the stolon is often called a runner. Some authors treat a runner as a specialized form of the stolon, defining the runner as having highly reduced or minute scales. This contrasts with the term stolon, which, in the strictest sense, requires the structure to have has some leaf blades. Therefore, a stolon would be defined narrowly as a photosynthetic unit with green tissues, whereas a runner would be not be able to sustain itself, being a mere connector between the mother plant and its offspring. However, consumers of these terms need not make any such distinction.
A mother plant produces stolons often in several compass directions. The plantlets produced are termed ramets [from the Latin ramus, branch]. The stolon connects the mother plant with each ramet and thereby provides the pathway for a flow of nutrients and water to the new plantlet, or even some nutrients from the plantlet back to the mother plant. But that physical connection is eventually severed or becomes dysfunctional as the plantlet develops its nutritional independence. After the stolons wither and are broken, a mother plant is encircled by satellite plantlets; these soon grow larger, filling in any space between the plants. In this way, stoloniferous species usually colonize open ground by forming a continuous ground cover, and thereby can exclude other species by crowding them out. Thus, many species that have been domesticated as turfgrasses and ground covers are stoloniferous, forming dense clonal monocultures.
For lawns, a number of stolon-bearing plants come to mind. The most common one is bermudagrass, Cynodon dactylon (Poaceae), which can form long stolons that cross over concrete and bricks on the opposite side, there forming adventitious roots from nodes that contact the soil. More formal lawns of country clubs and estates utilize St. Augustine grass, Stenotaphrum secundatum; with this species, you can easily see that the green blade on stolons are highly reduced in comparison with the more typical leaves of tufted plants. Another example-for the homeowner who wants to be different-is the stoloniferous Dichondra (Convolvulaceae), a dicotyledon, which never needs to be moved but does requires special handling to avoid formation of bare patches.
Growing as weeds in Westwood lawns are several other stoloniferous dicotyledons, including the undesirable sorrel, Oxalis corniculatus (Oxalidaceae), and the very useful, nitrogen-fixing, leguminous white clover, Trifolium repens (Papilionaceae).
In border gardens can be found strawberry plants as well as other rosaceous relatives, such as Indian strawberry, Duchesnia indica, which has yellow rather than white or rose-colored flowers. Geum reptans, also with yellow flowers, is another example. Of course, the term repent means creeping or prostrate, and often is used to describe stoloniferous plants. Look also for plants that bear the specific epithet stolonifera, i.e., "stolon-bearing." Among saxifrages (Saxifragaceae) are some stoloniferous species, such as Saxifraga stolonifera, a shade-loving woodland species that forms thin red stolons during spring growth.
Among aquatic plants that clone are some highly successful species that reproduce asexually via stolons. The floating aquatic water hyacinth, Eichhornia crassipes (Pontederiaceae), forms thick, white stolons, enabling this species to clone at an alarmingly high rate in tropical waterways. Other wideranging and highly competitive stoloniferous floating aquatics are water soldier (Stratiotes aloides, Hydrocharitaceae), water-lettuce (Pistia stratiotes, Araceae), Hydrocharis morsus-ranae (Hydrocharitaceae), and Potentilla palustris (Rosaceae). Numerous species of wetlands are likewise stoloniferous. Some would classify stolons instead as rhizomes, when they occur below the water surface, and, indeed, rhizomes are virtually the same as stolons except that they tend to have shorter internodes.
The cultivated white (Irish) potato, Solanum tuberosum (Solanaceae), surely planted somewhere in a Westwood garden, forms its edible tubers at the tip of stolons. The stolons grows from axillary bud at the base of the shoot, and its tip, forming a tuber, becomes buried in the leaf litter and loose soil around the plant, where the starchy tuber develops. This type of tuber is referred to as a shoot tuber.
If we want to finish with the analogy of mother with babies around her, no better example exists than species of Sempervivum (Crassulaceae), bearing the common name hens and chicks. This is a leaf succulent life form, hailing from mountainous regions of Eurasia. Sempervivum produces a mat of rosette plants around the initial mother plant. Such runners belong in a Westwood rock garden, and certainly along nearby Olympic Avenue, in honor of some past UCLA Olympians.
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