Ethylene gas promotes fruit ripening; in a classic example of a positive feedback loop, ripe fruit causes the production of more ethylene gas, which promotes more fruit ripening (this is the origin of the phrase "one bad apple spoils the bunch!")

Cytokinins stimulate cell division and cytokinesis, and they can delay senescence (deterioration due to age, as in the case of cells). Gibberellins promote cell growth, and also promote stem and leaf elongation. Abscisic acid (ABA) promotes seed dormancy, inhibits plant growth, and closes plant stomata during water shortages. 

Auxins play a number of crucial roles in plant growth, behavior, and development. They were the first major class of plant hormone to be discovered and are present in all parts of a plant in varying concentrations. Some of their more notable functions include causing phototropism, enhancing apical dominance, and softening the cell wall to stimulate cell elongation and growth. Indoleacetic acid (IAA) is a naturally occurring auxin, though some synthetic auxins are used as weed killers.

Ethylene gas is a plant hormone that promotes fruit ripening in a positive feedback loop. Ripe fruit produces more ethylene gas, which promotes more fruit ripening, and so on. It is not a kind of auxin.

In addition to growing in height, woody plants also grow in thickness. This is the function of lateral meristems. Lateral meristems are comprised of the vascular cambrium, and by cork cambrium that form vascular cylinders. The vascular cambrium adds layers of secondary xylem and phloem (wood), whereas the cork cambrium replaces the outer epidermis with a thicker and tougher layer called periderm.

Cellulose, a polymer of glucose, is the main component of plant cell walls. 

Collagen is found in the connective tissues of animals. Chitin is found in the cell walls of fungi. Actin and myosin are the proteins responsible for contraction in muscle cells; actin is also a microfilament in the cytoskeleton. Peptidoglycan is found in the cell walls of bacteria.

The plant cell wall is made of cellulose, which makes it rigid. The cell wall offers structural support and protection to the cell and it’s rigid nature maintains the regular shape of the cell. The cell wall is also permeable and allows small molecules, proteins, and nutrients to pass, including water and carbon dioxide.

Plant cells contain one very large vacuole that occupies a large percentage of the cell space. The central function of this vacuole is to maintain the turgor pressure of the cell through the continual transport of water and ions in and out of the cell, as needed. Vacuoles store and transport water, waste, and ions too.

Plant cells contain most of the membrane-bound organelles that animal cells do; they do not have cilia, and only some plant cells contain centrioles. Plant cells have additional organelles that animal cells do not, namely chloroplasts and cell walls, for photosynthesis and support, respectively. Virtually all cells have cell membranes.

Stomata are the microscopic pores on the surface of leaves. Their function is to allow carbon dioxide into the plant while also allowing oxygen to leave the plant.

The correct answer to this question is Only plant cells have cell walls

Animals notably do not have cell walls. Animal cells, not plant cells, have plasma membranes that contain cholesterol. Both animals and plants have cells with mitochondria and have ribosomes attached to the endoplasmic reticulum. Also only animal cells contain flagella and plant cells do not contain flagella at all.

The Calvin Cycle (dark reactions) take place in the stroma of the chloroplasts, which is the aqueous space inside the organelle.