Desert plant populations have evolved sophisticated physiological beha

Desert plant populations have evolved sophisticated physiological behavioral traits that aid survival in arid conditions. Some send out long, unusually deep taproots; others utilize shallow but widespread roots, which allow them to absorb large, intermittent flows of water. Certain plants protect their access to water. The creosote bush produces a potent root toxin which inhibits the growth of competing root systems. Daytime closure of stomata exemplifies a further genetic adaptation; guard cells work to minimize daytime water loss, later allowing the stomata to open when conditions are more favorable to gas exchange with the environment. Certain adaptations reflect the principle that a large surface area facilitates water and gas exchange. Most plants have small leaves, modified leaves (spines), or no leaves at all. The main food-producing organ is not the leaf but the stem, which is often green and non-woody. Thick, waxy stems and cuticles, seen in succulents such as cacti and agaves, also help conserve water. Spines and thorns (modified branches) protect against predators and also minimize water loss.

1. The passage refers to the spines and thorns of desert plants as

I. genetically evolved structural adaptations that protect against predation
II. genetic modifications that aid in the reduction of water loss
III. structures that do not participate directly in food production

(A) I only
(B) III only
(C) I and II only
(D) II and III only
(E) I, II and III



2. The author suggest that the guard cells of desert plants act to do which of the following?

I. Facilitate gas and water exchange between the plants and their surroundings
II. Cause the stomata of desert plants to remain closed during daytime hours
III. Respond to sudden, heavy rainfalls by forcing the plants’ stomata to open

(A) I only
(B) II only
(C) III only
(D) I and II only
(E) I, II, and III



3. The passage suggests that which of the following weather-related conditions would most benefit plants with shallow root systems?

(A) An unusually prolonged drought
(B) A windstorm
(C) A flash flood
(D) A light spring rain
(E) A winter snowfall



4. The adaptations of desert plants to their environment would tend to support the statement that

(A) the rate of genetic evolution is greater in the desert than in more temperate surroundings
(B) structures in a plant which usually perform one function may, under certain conditions, perform different functions
(C) while the amount of leaf surface area is critical for a desert plant, it is much less so for plants in most other environments
(D) desert plants do not have many physiological and behavioral traits in common with other plants
(E) desert plants could probably adapt to life in a variety of harsh ecosystems