Symbolism is something that is used very strongly in this novel to refer to some of the main characters. Leo, for example, is referred to at various points by the adult characters he serves as Mercury, recalling the Roman messenger god. This impression is also reinforced by the tremendous heat that summer and how mercuy is an element that reacts very quickly to heat or cold. Leo certainly does act as a messenger, even though he may not be fully aware of the import of his role, and this is an apt symbol for his role in the novel.
Marian Maudsley, however, is symbolised in the very ambiguous atropa belladonna plant, or the deadly nightshade, that Leo discovers in the grounds at the end of Chapter 2. Note how Leo describes it as he comes across this beautiful but poisonous plant:
I felt that the plant could poison me, even if I didn't touch it, and that if I didn't eat it, it would eat me, it looked so hungry, in spite of all the nourishment it was getting.
This symbol is one that is associated with Marian, who is of course incredibly beautiful. However, the lust and attraction that Leo himself does not fully understand as a young boy entering puberty is highlighted in the "poison" of the nightshade plant. The plan therefore acts as a symbol for Marian in order to highlight the dangers of sexual temptation. Just as Ted dies as a result of tangling with the nightshade, Leo suffers as well after tangling with it in the form of Marian, therefore presenting the dangers of arousal and lust.