The tree and snake motif may be understood as follows:
The dragon represents the negative mother-imago and thus expresses resistance to incest or fear of it. Dragon and snake are symbolic representations of the fear of the consequences of breaking the taboo and regressing to incest. It is therefore understandable that we should come over and over again upon the motif of the tree and the snake. Snakes and dragons are especially significant as guardians or defenders of the treasure
CW5 ¶ 395TREE ENTWINED BY THE SNAKE
The tree entwined by the snake may therefore be taken as the symbol of the mother who is protecting against incest by fear. This symbol is frequently found on Mithraic monuments. The rock with a snake coiled round it has a similar meaning, for Mithras was born from a rock, as was the god Men
CW5 ¶ 396