'What makes it possible for weeds to grow through a two-inch-thick asphalt sidewalk?'

Well?

Hercules in the Garden

Q. What makes it possible for weeds to grow through a two-inch-thick asphalt sidewalk?

A. The amazing strength of plants, from the mighty oak to the slenderest shoot or root, involves their inexorable growth and expansion by cell division. But it also depends on access to a water supply and on two interacting processes for handling water: osmosis and imbibition.

Water enters a plant’s roots and moves through its cells by way of osmosis, diffusing through cell membranes from regions of weak concentrations of dissolved materials to regions of higher concentrations.

Imbibition is the swelling of plant tissues to several times their original volume. It comes about as the electrical charges on materials in suspension within the cells of the tissue make these substances interact with the polarized water molecules, which perform like tiny magnets. The suspended materials include minerals, cellulose and starches.

Imbibition is responsible for the germination of seeds in the first place, and can continue within a plant’s growth cells, leading to a tremendous internal pressure. Over short periods and long — sometimes over many years, in the case of tree growth — the result can be enough power to break sidewalks or even boulders.

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[via C. Claiborne Ray's "Q&A" column in the February 20, 2007 New York Times Science section]