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Adaptations to Fulfill Basic Needs
the strength to support themselves and the weight of their leaves. The seeds of many sun-loving species have adapted to avoid germination under a leaf canopy by responding to light quality differences between direct sunlight and that which passes through the shading leaves (see chapter 1). But such a system restricts sun-loving species to places devoid of larger plants or periods when surrounding deciduous trees have lost their foliage. In an effort to grow the shortest distance to the brightest available light, the stems of some sun-loving species grow away from shade in a horizontal position; in other species the stems grow vertically but use nearby rigid objects as supports. SPREADING STEMS
When stems recline on the soil surface or grow below ground, they have no need to spend energy or nutrients on the construction of metabolically expensive strengthening tissues. Thus, they are able to direct all of their resources into a burst of rapid, primary growth that carries the leaves into more favorable illumination. Horizontal stems growing aboveground are called runners, or stolons (Latin for “shoot”); those growing underground are rhizomes (Greek: rhizoma, “root”). From nodes on stolons and rhizomes, roots and upright shoots develop, the latter from axillary buds. Roots arising from the sides of stems, as these do, are described as adventitious roots (Latin: adventicius, “coming from outside”). In a broad sense, the word applies to any root emerging in an unusual position, such as those that grow on stem or leaf cuttings. Stolons generally emerge from near the crown of a plant, bend under their own weight, touch the soil, and develop plantlets at their tips. These, in turn, send out more runners in a stepwise fashion claiming an everwidening circle of ground. Thus, a strawberry patch may grow from a single, spreading plant by this natural method of vegetative propagation. It is this same stoloniferous habit that makes some ornamental species well suited for use as ground covers. The proliferation of underground rhizomes is less apparent, but no less effective, in the occupation of large surface areas of soil. As long as favorably lit places are open for encroachment, the relentless advance of perennial stoloniferous and rhizomatous species may continue indefinitely. A gardener who has cleared a bed of such species knows how persistent they are when, for years after his labors, fragments of broken stems send up sprouts.
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