68. In biology lab, your teacher places petri dishes on the lab table. Upon examination, you observe that there are lumps of plant tissue, and you are told that they have been grown artificially and can produce new plants. Each lump is a(n)__
(A) tetraploid zygote；
(B) stigma；
(C) amnion；
(D) callus.

64. The _____ and _____ of a plant's leaf help conserve water in a plant. (A) root hairs； stomata； (B) Casparian strip； cuticle；(C) Casparian strip； endodermis；(D) stomata； cuticle.

65. If water is abundant, a plant's guard cells will____ and the stomata will_____. (A) swell； close；(B) collapse； open；(C) swell； open；(D) collapse； close.

66. The whorl of a flower that consists of all the sepals is the__? (A) stamen；(B) calyx；(C) carpel；(D) corolla.

67. The plant hormone that stimulates cell division in seeds and fruits is__? (A) auxin；(B) cytokinin；(C) abscisic acid；(D) ethylene.

69. Which phrase defines smooth muscle? (A) used for slow, involuntary movements；(B) used to move bones；(C) beats the heart；(D) shows striations.

70. The systems that coordinate communication are__? (A) skeletal and muscular；(B) circulatory and respiratory；(C) nervous and endocrine；(D) digestive and circulatory.

71. Which of the following best describes how a neuron fires? (A) Na+ ions cross the plasma membrane, initiating a wave that travels down the axon；(B) Vesicles carry neurotransmitters from the nucleus to the other end of the neuron；(C) Na+ ions enter one end of the neuron and diffuse to the other end, down the axon；(D) Neurotransmitters enter one end of the neuron and diffuse to the other end, down the axon.

72. If you had a friend who was unable to see color, you would expect that your friend's eyes had defective__? (A) cone cells；(B) rod cells；(C) cornea；(D) iris.

73. Observing a patient who has been given an experimental drug, you notice that it constricts blood vessels and slows digestion, making its effects similar to those of__? (A) calcitonin；(B) epinephrine；(C) insulin；(D) glucagon.

74. A fast-twitch muscle fiber__? (A) uses ATP quickly in short, fast contractions；(B) has many capillaries；(C) is rich in myoglobin；(D) works aerobically.