In the New Yorker cartoon the rock stands for "stability". In the dictionary stability means: the state or quality of being stable; strength to stand without being moved or overthrown; as, the stability of a structure; the stability of a throne or a constitution. Also listed were synonyms of stability, which were balance, composure, proportion, steadiness, and symmetry. I think that a rock is an easy metaphor for stability because a rock possesses the quality of not easily being changed or moved. If there is a large rock, a mountain for example, and I look at that rock every day there is not going to be any change in that rock from day to day. I could even extend that period and come back to the mountain once a year, or even every ten years and the physical characteristics of that mountain are not going to change, save for the rare chance of a large natural disaster. A rock is very stable and does not go through the processes of change.
The tree is the metaphor for growth and change and this also translates very easily. Trees are constantly in a state of change due to natural causes. In the winter many trees lose all of their leaves and appear very bare and open. When the season begins to change to spring, the trees begin to blossom or "spring into full bloom". During the summer the tress are generally full of green leaves and bear fruit. In the fall the leaves begin to change colors and fall off of the limbs. This whole process seems very similar to the "four mythoi" diagram from Northrup Frye. In the winter is irony and it seems ironic that beautiful green trees would be bare and leafless and not as beautiful in the winter. In the spring is a comedy and I can relate that to happiness and new comings. The new blossoms are waiting to come out and it is a time of joy. Here in the United States more babies are born in the summer than any other time of year. Most fruit-bearing trees also bear fruit in the summer, this seems to be no coincidence that Frye would put romance in the summer because there is nothing more romantic than the fruits of love.