In watersaturated soils oxygen is scarce

For some plants, the environmental challenge is the opposite of that faced by xerophytes: too much water. Some plants live in environments so wet that the diffusion of oxygen to their roots is severely limited. Since most plant roots require oxygen to support respiration and ATP production, most plants cannot tolerate this situation for long.

Some species, however, are adapted to life in a water-saturated habitat. Their roots grow slowly and hence do not penetrate deeply. Because the oxygen level is too low to support aerobic respiration, the roots carry on alcoholic fermentation (see Chapter 7), which provides ATP for the activities of the root system. This adaptation explains why their growth is slow.

The root systems of some plants adapted to swampy environments have pneumatophores, which are extensions that grow out of the water and up into the air (Figure 40.11). Pneumatophores have lenticels and contain spongy tissues that allow oxygen to diffuse through them, aerating the sub-

Examples Pneumatophores Roots
Pneumatophores are root extensions that grow out of the I water, under which the rest of the roots are submerged.

40.11 Coming Up for Air The roots of the mangroves in this tidal swamp obtain oxygen through pneumatophores.

Mangrove Aerenchyma

Cells obtain oxygen through projections into the open channels of air-filled aerenchyma tissue.

Vascular bundle

40.12 Aerenchyma Lets Oxygen Reach Submerged Tissues The scanning electron micrograph, a cross section of a petiole of the yellow water lily, shows the air-filled channels of aerenchyma tissue.The cells that line these channels obtain oxygen by extending projections into these channels.

Cells obtain oxygen through projections into the open channels of air-filled aerenchyma tissue.

Vascular bundle

40.12 Aerenchyma Lets Oxygen Reach Submerged Tissues The scanning electron micrograph, a cross section of a petiole of the yellow water lily, shows the air-filled channels of aerenchyma tissue.The cells that line these channels obtain oxygen by extending projections into these channels.

merged parts of the root system. Cypresses and some mangroves are examples of plants with pneumatophores.

Submerged or partly submerged aquatic plants often have large air spaces in the leaf parenchyma and in the petioles. Tissue containing such air spaces is called aerenchyma (Figure 40.12). Aerenchyma stores oxygen produced by photosynthesis and permits its ready diffusion to parts of the plant where it is needed for cellular respiration. Aerenchyma also imparts buoyancy. Furthermore, because it contains far fewer cells than most other plant tissue, respiratory metabolism in aerenchyma proceeds at a lower rate, and the need for oxygen is much reduced.

Thus far we have considered water supply—either too little or too much—as a factor limiting plant growth. Other substances also can make an environment inhospitable to plant growth. One of these substances is salt.

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