Several points to consider. First, what is the format/style here (APA, MLA, CSE or other)? Second, I’m not a biologist, so writing the discussion and conclusion is outside my skill set. Third, what I can do is look at what you have here and try to provide some suggestions.
 
Perhaps your approach should be to write according to what you’ve explained here in your email: you did the research, you did find an answer (plant treated with 10 mg/L is an anomaly) and then you can mentioned several possible reasons why you found this result. If you think that this is not the answer, then you can propose several possibilities (i.e., could be multiple answers). 

Consider the possibility that this assignment might not have one answer and everyone who did this same experiment might get different answers. 

Your professor said that the dwarf would grow more, but did he/she also say “under what conditionsâ€? 

Consider this: you and the other students apparently used the same materials and methods that you describe in the text, but the results differ….maybe that’s the answer and then each student explains why his/her results differ from the professor’s statement that the dwarf will grow more (it might grow more for some students, and not for others). 

YOUR answer then is in why the hypothesis (dwarf grows more) proved untrue for YOUR particular experiment. 
