In lines 13-17, why does the author compare tree roots with growing salt crystals?

Although the earth was formed about 4,500 million years ago, human beings have inhabited it for less than half a million years. Within this time, population has increased bugely and people have had a vast (1) _______ upon the earth. They have long been able to (2) _______ the forces of nature to use. Now, with modern technology, they have the power to alter the balance of life on earth.

Reports back from the first astronauts helped dispel the dangerous (3) _______ that the world had no boundaries and had militless resources. (4) _______, ecologists have shown that all forms of life on earth are interconnected, so it (5) _______ that all human activity has an effect on the natural environment.

In recent years, people have been putting the environment under stress. As a result, certain (6) _______ materias such as timber, water and minerals are beginning to (7) _______ short. Pollution and the (8) _______ of waste are already critical issues, and the (9) _______ of the environment is fast becoming the most pressing problem (10) _______ us all. The way we respond to the challenge will have a profound effect on the earth and its life support (11) _______

However, despite all these threats there are (12) _______ signs. Over the past few decades, the growth in population has been more than matched by food production, indicating that we should be able to feed ourselves for some time yet.