Chapter 31. Plant Growth and Development

The structures and functions of root systems are diverse.

How a plant’s roots are distributed in the soil has a tremendous impact on the ability of the root system to supply the shoot with water. Some plants produce very deep roots that provide access to groundwater that is independent of the variability of rainfall. For example, roots have been observed in mine shafts and caves more than 50 m below the soil surface. Producing and maintaining these non-photosynthetic structures is costly for the plant.

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At the other extreme are plants that produce shallow and spreading root systems. For example, barrel cacti produce roots that penetrate less than 10 cm into the soil but extend out several meters from the base of the plant. These roots cast a wide but shallow net to capture as much water as possible from intermittent rain showers.

Some plants produce distinctive roots whose principal role is not the absorption of water and nutrients from the soil (Fig. 31.18). For example, climbing plants such as poison ivy produce roots along their stems that allow them to adhere to the sides of trees (Fig. 31.18a). The conspicuous prop roots produced by some tropical trees provide mechanical stability that allows these plants to grow tall despite their slender stems (Fig. 31.18b).

FIG. 31.18 Root diversity. (a) Climbing roots of poison ivy; (b) prop roots of a Pandanus tree; (c) storage roots of yams; and (d) breathing roots of black mangrove (shown at low tide).

Many plants produce swollen roots that store resources such as starch (Fig. 31.18c). Plants with belowground storage can produce new leaves and shoots quickly following either damage or a period of drought or cold that has killed the shoots. Cassava and sweet potato are examples of economically important species with storage roots.

Perhaps the most unusual rooting structures are the “breathing” roots produced by trees that grow in water. For example, black mangroves produce pencil-sized roots that extend vertically upward out of the sandy soil (Fig. 31.18d). Breathing roots do not actually breathe; instead, they contain internal air spaces that provide an easier pathway for oxygen to diffuse into the roots than through the waterlogged soil. For this reason, breathing roots must extend above the surface of the water.