Biology Now, 2e

The Dirt on Black-Market Plants ■ 291

In addition to being food, plants are valuable

for many other reasons. Many organisms live on

or in plants, or in soils largely made up of decom-

posed plants. By soaking up rainwater in their

roots and other tissues, plants prevent runoff

and erosion that can contaminate streams.

Plants (and algae) also recycle carbon dioxide

and produce the oxygen we breathe.

Many of these traits are adaptations to the

challenges of living on land (Figure 16.6). The

greatest of those challenges is how to obtain and

conserve water. The first plants, the ancestors of

algae from which they evolved, plants use chloro-

plasts in order to photosynthesize. Most photo-

synthesis in plants takes place in their leaves,

which typically have a broad, flat surface—a

design that maximizes light interception.

Because plants are producers, they form the basis

of essentially all food webs on land. They may

not be as cute, fuzzy, or exciting as animals, but

animals wouldn’t exist without them: nearly all

organisms on land ultimately depend on plants

for food, either directly by eating plants or indi-

rectly by eating other organisms that eat plants.

Figure 16.6

Moving to land brought unique challenges to plants

Terrestrial plants evolved in response to challenges that their aquatic ancestors had not faced, which resulted

in adaptations designed to slow dehydration, provide support and anchoring, and enable photosynthesis and

nutrient uptake.

Q1: In what ways are terrestrial plants and their aquatic ancestors the same? Give at least two similarities.

Q2: In what ways do terrestrial plants and their aquatic ancestors differ? Give at least two differences.

Q3: Would you predict that aquatic plants (which have secondarily evolved to live in water) would be more like plants in a

rainforest or more like desert plants? Explain your reasoning.

Algae (aquatic) Plants (terrestrial)

● Photosynthesis occurs and CO 2 is absorbed

throughout the organism.

● Photosynthesis occurs and CO 2 is absorbed primarily in the leaves.

Air enters leaf cells through minute openings in the cuticle. Many

plants have more elaborate pores (stomata) that open and close to

regulate the flow of gases into and out of the leaf.

● Roots absorb water and minerals from the soil.

● Roots anchor and support the plant within the ground. Lignin and

vascular tissues help support the plant aboveground.

● The waxy cuticle holds in moisture to keep plant tissues from drying

out, even when exposed to sunlight and air throughout the day.

● Water and minerals are absorbed by the whole

organism.

● Water supports the whole organism.

● Dehydration is not an issue in an aquatic environment.