How can rhododendrons grow on such materials?
Students of plant nutrition have learned a great deal about the root activity of ordinary plants in ordinary loam soils, where the clay or humus particles conserve moisture and serve as sites for pick-up of nutrients. Here, on the other hand, we are concerned with extraordinary plants, i.e., rhododendrons and other members of the Heath Family, which grow on gravel, volcanic ash, granite grit, and other materials containing little or no clay or humus. I do not know of any fully adequate explanation of their mode of existence, but we can perhaps get some inkling by looking at their roots. These are usually a shallow mat and include a large number of small rootlets. This type of root system probably protects soil moisture and, when cool, it tends to trap water from any vapor which rises from below. Also, the numerous root fibers provide a relatively large amount of root surface area, and this may be one of the reasons why they can take up enough water and nutrients from coarse soil which
