Question Video: Understanding the Unequal Growth of a Plant Stem | Nagwa Question Video: Understanding the Unequal Growth of a Plant Stem | Nagwa

# Question Video: Understanding the Unequal Growth of a Plant Stem Biology • Second Year of Secondary School

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The graph shows how the shaded and illuminated sides of the tip of a plant stem change over time. How will this affect the growth of the plant?

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### Video Transcript

The graph provided shows how the shaded and illuminated sides of the tip of a plant stem change over time. How will this affect the growth of the plant? (A) The plant will grow toward the light source. (B) The plant will grow away from the light source. (C) The plant will grow irregularly, which will stunt its development. Or (D) there will be multiple side shoots.

Let’s start by taking a careful look at this graph so we can answer the question. The 𝑥-axis represents time, measured in minutes, and the 𝑦-axis represents a length increase in the cells in the stem tip, measured in millimeters. Therefore, the graph shows the growth over a short space of time of the two different sides of the tip of a plant stem: one that is illuminated by light and one that is not. We can see pretty clearly that the side of the stem that is shaded and not illuminated by light grows a lot more than the side that is illuminated.

What happens if one side of a plant stem is growing faster than the other side? If cells on one side of the stem elongate while the cells on the other side do not grow as fast, the stem begins to bend away from the direction of the elongating cells. This is because upward growth is restricted by the cells that are not elongating so much. As the cells on the shaded side of the stem are lengthening comparatively more than the cells on the illuminated side, the stem bends toward the source of illumination. This is how plants are able to grow toward light, through a process called phototropism. We can deduce that the differences in growth between the shaded and illuminated sides of a plant stem, which have been illustrated in the graph, cause it to grow toward a source of light.

So, the correct answer to this question is (A). The plant will grow toward the light source.

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