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Unable to find moisture beneath the heavy flagging, the tree's outer roots had atrophied while inner ones had multiplied and massed under the "pot' Here they became self-constricting, and entirely dependent on artificial drink and food, which were not enough. The solution: to drill holes and insert short lengths of pipe down through the flagstones, spaced widely around the "pot"; then, by frequent watering and feeding, to coax the horsechestnut's root system back outward to a normal pattern. (This system, with sieve caps over the pipe inserts and a cutting tool to clear the pipes when rootlets clog them, as they will, can be used to preserve feature trees rooted where a driveway must go.) In the tree's ascending column of water are dissolved minerals from the soil. Chief of these are nitrogen, phosphorus, and potassium, which the tree must have, besides sugars and starches, in forms synthesized by leaf chemistry. At this peak point, in the leaves, the tree's water content becomes enriched sap. Now it must be redistributed downward to impart growth, energy and tensile strength to all parts of the tree. To see how this is done we must re-examine the tree's water column, and now we find that it is a two-way affair. On the way up, the moisture takes an inner course through deep layers of xylem or sapwood cells, just outside the heartwood (which is old xylem cells grown inert). Surrounding this thick cylinder of sapwood is a thin outer one composed of tubular phloem cells through which the enriched sap is conducted earthward. Where the two parts of this pipe-within-a-pipe touch is called the cambium (exchange) layer. From it extend lateral fissures called medullary rays, through which both water and sap are transferred inward. This dual circulatory system (See Fig. i.) is present all the way from the slimmest leaf stem down through twig, branch, limb, and sturdy trunk into |
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