Tropism

In the syllabus and the exam

Tropism is the umbrella term introduced at the start of the responses in plants topic, before candidates study specific examples such as phototropism and geotropism individually, and questions on tropism in general often test whether candidates understand the positive/negative naming convention correctly. A response is called positive when the plant part grows towards the stimulus and negative when it grows away from the stimulus, and candidates must apply this consistently: shoots typically show positive phototropism (growing towards light) but negative geotropism (growing away from gravity, that is, upward), while roots show the reverse pattern for each. Exam questions commonly ask candidates to define tropism in general terms before asking about a specific type, so a vague answer that omits the idea of directional growth in response to a stimulus will not earn full marks even if the specific example given afterwards is correct. A frequent error is confusing tropism, a growth response that takes place over hours or days and results in a permanent change in the plant's form, with a much faster nervous response such as a reflex action seen in animals. Candidates should also avoid describing a tropism as movement of the whole plant, since it is specifically an unequal growth response of cells on either side of the responding organ. A further nuance sometimes tested is that different plant organs can respond to the same stimulus differently, so a well-prepared candidate should always specify which organ is being discussed rather than making a general claim about how an entire plant responds to light or gravity.

Example

Defining the general term, a candidate writes: 'A tropism is a directional growth response of a plant towards or away from a stimulus such as light, gravity, or water, and it is described as positive when growth is towards the stimulus and negative when growth is away from it.'

Related terms

PhototropismGeotropismAuxin

Source:SRC-DSKP-EN