What is the fig tree branch metaphor?

Just as the fig tree throws out new sprigs in its growth, every new petal delicately formed, pure and transparent-like in colour—the lower branches and leaves becoming hardened from atmospheric exposures become firm and strong and repel the storms, which beat against them. 

Now and then a sprig may be torn off through the positive force, but the young twigs hold their position, holding to the trunk. 

In this wise the spirit of (hu)man grows upward, every faculty reaching with its perceptiveness into space—

The spirit gradually ascends to the topmost pinnacle of morality, pure love and the highest principles of virtue.