The Life of a Leaf by Steven Vogel

Author:Steven Vogel
Language: eng
Format: epub
Publisher: The University of Chicago Press

9 Cleaning Surfaces

Fall has now turned to winter, and my annual raking again impresses me with the enormous area of leaf put out every year by a modest number of suburban trees. These relatively inefficient solar panels provide all the energy a tree can invest in growth, reproduction, and dispersal—the three central concerns of every organism that has ever lived. That takes real acreage: in a forest, aggregate leaf area greatly exceeds ground area.

Most broad leaves weigh very little: paper thin, a great pile of freshly fallen ones can be bagged and carried with little effort. An ordinary wheelbarrow, suitable for carrying dirt, holds all too little. A high-sided yard cart does better, and with such a light load one doesn’t get back pains from its ergonomically awful crosswise handle. I once had the idea that the cost of operating a water heater could be reduced by running the input through a pile of composting leaves—the temperature of an active pile runs well above that of the surrounding air. A rough calculation disabused me of the idea, by showing that the elevated temperature came more from the pile’s self-insulation than from actual heat generation, so low was its combustible mass.

Consider an odd implication of all that lightweight photosynthetic area. What if it rains on leaves when there’s very little wind? Water is dense stuff, a bit denser than the leaves themselves. Wet leaves outweigh dry leaves, perhaps not as dramatically as a wet towel outweighs a dry one, but by appreciably more than their unwet weight. (“Dry weight” has another meaning in the botanical business, hence my use of the circumlocution “unwet.”) So the sheer weight of soggy or water-coated leaves might present a structural challenge. In addition, a coating of water will hang around, and water provides a better biological medium than does air. It’s likely to contain all sorts of spores and various microcreatures that lunch on leaves, ones that grow larger or more numerous over time.

On both accounts, weight and contamination, leaves ought to prefer that water run off them. Since few will be perfectly horizontal, especially when burdened with the extra weight of raindrops, that’s most often what happens. Water runs off leaves like it does off the proverbial duck’s back—and for the same reason. Both are quite hydrophobic. In all the talk about surface tension in chapter 7, nothing was said about the most obvious surface of all: the outside surface of a leaf.

How Hydrophobic Can You Get?

Surface tension, in the guise of capillarity, came up when we considered the height that water might rise in a glass tube or xylem conduit to which it feels some attraction. And surface tension came up again in explaining why air doesn’t enter the open top ends of the water columns that run up trees. And it appeared yet again in the explanation of why a lot of negative pressure might be needed to draw water out of soil. In each of these instances, water has a high

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.