Final answer:
Wents would have observed increased growth on the side with the agar block due to the auxins within it, promoting primary growth and cell elongation influenced by phototropism. The correct answer is option b.
Step-by-step explanation:
If Wents placed his agar blocks only halfway on the cut surfaces of grass shoots with their tips removed, the observer should have observed increased growth on the side with the agar block (Option B). This is due to auxins, a group of plant hormones responsible for regulating growth, which are known to be more concentrated at the tips of plant shoots. The presence of these hormones in the agar block would stimulate growth more on the side where the block is placed, compared to the side without the block. As grasses exhibit primary growth from the apical meristem, the tip of the shoot, this area is vital for the shoot's elongation and growth.
Grasses, such as the wheat apical meristem, do not engage in secondary growth but rely on primary growth which occurs at the apex of the shoot. It encourages the plant to grow upward, known as negative gravitropism, and roots to grow downward, referred to as positive gravitropism.
Phototropism also plays a role in the growth of the grass shoots. This directional growth of a plant in response to light relies on the presence of auxins that move to the shaded side of the plant, causing cells there to elongate more than those on the light side. Using agar blocks in experiments has shown that the substance responsible for this growth, which Boysen-Jensen identified as a chemical signal, moves through the plant. The hormone indole acetic acid (IAA) is the one causing the phototropic response to light, primarily influencing cell elongation on the shaded side.