Since there is also a very clear difference between this background response and the positive chemoattractants, it does not invalidate the experiment. In fact, interpretations of many types of experiments must typically factor in background level responses. Succinic acid shows a very weak response. Examine the chemical structures provided by your teacher. All of the test substances are four-carbon acids.
Succinic acid lacks the hydroxyl group that is present on the others.
Since it gives a weak response, the hydroxyl group appears important for recognition. Presumably, membrane surface receptor sites on the sperm have different affinities for the different chemicals, and this is based on their structure. However, membrane receptors associated with the C-Fern chemotactic response have not yet been isolated or described, so their existence is only hypothetical. L-malic acid is a common Krebs cycle intermediate and is likely a significant component of the discharge from receptive archegonia. This discharge, which may also contain other unknown chemoattractants, allows sperm to locate receptive archegonia containing eggs.