Narrator: Listen to part of a lecture in a botany class. The professor will discuss cacti.

The cactus is one example of the way that plants adapt to extreme conditions of climate. cactus like this one has the same basic [bad word] as all other plants, but the function of leaves is carried out by the stems and branches of the plant.
Here are some more examples of the most familiar cacti — the barrel, the Saguaro, and the prickly pear. As you see, they all have stems, branches, or spines, but no leaves. In spring, they may have beautiful blossoms.
It is assumed that the predecessors of the modern cactus had leaves, but that during millions of years of changes in the climate, resulting in desert conditions, the cactus gradually adapted to the hotter, drier environment. The roots spread out and began to grow closer to the surface so that water could be absorbed more quickly. The roots and spongy or hollow stem of the cactus began to serve as a storage container for water, and the outer layer of the plant developed thick, waxy walls to prevent the water from draining out. Some varieties of cactus actually have ribbed folds that expand and contract depending on the volume of water stored inside the stem.
Although there are a very few members of the cactus family that retain their leaves, in most cacti, they have evolved into spines, needles, or hairs to protect the plant in areas where little green vegetation is available for foraging animals. In cacti without leaves, the stems and branches carry out the nutritive functions that usually take place in the thin leafed surfaces of other plants.

Narrator: Now get ready to answer the questions.