Epiphytic lichens can exert some negative actions on trees, since they
penetrate to different degrees into several tissues of the phytophore
. Evernia prunastri (L.) Ach. hyphae progress into xylem vessels of Fa
gus sylvatica, Quercus pyrenaica and Betula alba. This is on the basis
of a mechanism through which Lichens inject metabolic inhibitors in t
he xylem vessels, and, after their translocation in the xylem sap, the
y reach different organs of the phytophore and produce defoliation. In
this report, we have noted that usnic acid is more impermeant than at
ranorin through the chloroplast membrane of Quercus rotundifolia leave
s. On the other hand, we have also noted the degradation of chlorophyl
l in response to Lichen substances.