Hello I believe it should say "Leaves have waxy coatings which prevents them from losing water" Your usage of they is unnecessary. In regards to your second sentence it should say "Leaves have waxy coatings which allows them to save water" Note the addition of the letter s to the verbs following "which" in both sentences. In the case of both sentences it is proper to insert "them" because it is unnecessary in English to repeat your object in the same sentence and it will sound strange to a native speaker of the language (which I am by the way).
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