Question

How is the agar block shown in B different from that shown in A?

(A)B contains Auxin.
(B)B contains Cytokinin.
(C)B contains GA.
(D)None of the Above.

Answer 1

Hint: As we know that a few phototropism inventions were conducted by Charles Darwin. Where he noticed that the shoot tip of coleoptile tends toward the source of light. Most importantly, the bending did not occur on the tip but it created below it. When the tip is removed or covered with foil, it could be seen that the shoot tip did not bend anymore. Darwin concluded that “Some influence is transmitted from the tip to the more basal regions of the shoot thereby regulating growth and inducing curvature”.

Complete answer:
1. After Darwin, Boysen-Jensen investigated this issue more closely. He cut the tips off and placed a thin piece of silver or mica between the coleoptile and the lower part of the shoot. They found that the shot did not grow or curve toward the light. When they repeated the experiment using a block of gelatin/agar instead, the result was that the shoot grew and curved towards the light. Thus, he concluded that the ‘influence’, which Darwin mentioned in his studies was a water-soluble chemical capable of diffusing through the agar/gelatin from the tip. There it was produced to the lower, elongating part of the shoot where it had its effect.
2. This water-soluble chemical is termed as Auxin hormone.
3. The GA or Gibberellin hormone is responsible for the growths in the nodes of the plants. A node is a place on a stem where a leaf attaches, so gibberellins elongate the internodes. In dwarf plants, the hormone gibberellin is not present.
4. Cytokinin is such a hormone which delays the process of aging in the plants.
5. If we see the experiment above more closely, we can find that both Plants A and B have the auxin hormones which are responsible for elongated growth too for the plants. But the salient point is agar block is placed partially in plant A so the portion of shoot tip in plant A can be elongated. But due to agar blocks having covered most of the area in plant B, the shoot tip can’t be stretched as it was shown in plant A.
Hence, the correct answer is option(D).

Note: In this regard, we can say that both plants A and B have auxin and gibberellin. The hormone cytokinin is not included in the context. So, we can say that in the options we have not found any text or sentence which describes the agar blocks and their different placement on the shoot tip.