A saguaro cactus opens large white flowers at night. The flowers give off a strong scent and hold sweet nectar. How do these flower structures most help the cactus?

The seed heads of a burdock plant are covered in tiny hooks that catch on the fur of passing animals. How does this structure most help the plant?

A cherry tree grows sweet fruit around each hard seed. Birds eat the fruit, seeds and all, and then fly away. Which outcome of this behavior most increases the tree's reproductive success?

Emperor penguins raise one chick a year. The male keeps the egg warm while the female feeds at sea, and afterwards both parents bring food to the chick. How does this care most affect the pair?

In spring, male frogs at a pond call loudly each night, and females move toward the loudest callers. What is the main role of this calling behavior?

Grasses pollinated by wind make huge amounts of very light pollen, while plants pollinated by insects usually make sticky pollen and grow bright, scented flowers.

Six identical bean seedlings grew for three weeks with the same soil, water and temperature. The bright-light group reached a mean height of 18 cm. The dim-light group reached a mean of 9 cm. What do these data show?

Two apple trees of the same kind were planted in the same year. One grew in rich, well-watered soil and reached 4 m. The other grew in dry, rocky soil and reached 2 m. What best explains the difference?

A student touches a hot pan and pulls her hand back at once. Which path describes what happened in her body?

A moth flies toward a bright porch light at night. In this event, what does the light itself represent?

In a dim room, sensory receptors in a cat's eyes detect the low light and the cat's pupils become wider. Which part of this event is the response?

Pollen from one sunflower reaches the flower of another sunflower, and seeds form. What is most likely true of the seedlings that grow from those seeds?

A class grew four bean plants of the same kind with the same soil, water and light, but gave them 0 g, 2 g, 4 g and 6 g of fertilizer. The plants given more fertilizer grew taller. What best explains the height difference?

Seedlings grown from seeds of one parent plant got the same soil and 12 hours of light. Those watered daily grew tall, while those watered once a week stayed small. What caused this difference in growth?

Fish eggs in a hatchery hatched in a mean of 21 days at 8 degrees Celsius, 14 days at 12 degrees Celsius, and 10 days at 16 degrees Celsius. Everything else stayed the same. What pattern do the data show?

A farmer planted corn very close together in one field and farther apart in another. The crowded plants grew shorter and produced fewer ears of corn. What best explains this difference?

A honeybee visits many flowers of the same kind on one trip and carries pollen caught on its body hairs. How does this behavior affect the plants it visits?

Offspring produced by asexual reproduction show a wide range of variation, because they receive genetic information from two parents.

Two tomato seedlings were grown in identical soil, with the same water and the same light. One is a tall variety and reached 90 cm. The other is a dwarf variety and reached 30 cm. What best explains the difference in height?

A dog resting in a dark yard hears a faint rustling in the grass and turns its head toward the sound. Which receptors detected the stimulus, and what was the response?