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ANALYSIS OF PLANT ASH

From this ash we learn that less than five per cent of the dry weight of a rose plant is mineral matter derived from the soil, and that carbon, hydrogen, and oxygen comprise over ninety-five per cent of it. These come from carbon dioxide of the air and water from the soil and are chemically broken down and re-combined in the foliage. The resulting carbo­hydrates (same group as sugars and starches), together with amino-acids, are the true food of the plant, the basic building material of all wood, leaves, and blooms. All energy for growth and other plant activities is derived from its oxidation.

When plants are pruned hard, great quantities of food stored in the young branches are lost. When pruned after new shoots have grown there is similar loss. This late pruning may conveniently make roses bloom later than usual, but will give smaller blooms and less growth, because of this loss.

Loss of foliage, out of season, forces a plant to live on its reserves, for no more plant food can be made. Any new growth is weak, because it is made from inadequate food. The re­gaining of normal vigour is a slow process and should the new leaves be lost the plant is usually doomed. The most common causes of abnormal defoliation of roses are black spot, sun­burn, and spray-burn.

Carbon dioxide can be absorbed only by healthy leaves, in sunlight and when temperatures are sufficiently high. It is taken up by the stomata, quickly dissolved, and used in the making of complex substances by the sun's acting on chloro­phyll. Smoke and chemical fumes are injurious to plants; the former clogs the stomata, but is much less prevalent in these days of electricity than in former years, when it was impossible to grow any healthy vegetation in or near large cities. The chemical fumes seldom exist in sufficient concentration in the air to affect foliage, but are taken into the soil and are toxic to both soil bacteria and plant rootlets-probably an increasing menace.

From root-tip to leaf-tip every plant is an intricate system of water channels. The framework consists of cells, many of which contain as much as ninety-nine per cent of water. With such force do plants draw water from the soil through their roots that this moisture level can be maintained while they are reducing soil moisture to as low as three per cent in very light sandy loams. Heavier soils will not yield water to plants to this same extent.

Soils contain some uncombined carbon in the form of char­coal or soot; this is only very slowly, if ever, available to plants. Combined with oxygen, as carbon dioxide, some is carried into the earth by rain, some is liberated from lime (calcium car­bonate), some is generated in the process of decomposition of organic matter, and some is excreted by roots. With water it forms carbonic acid, and acts on many other compounds in the soil, but no carbon is absorbed by the roots. As carbon dioxide escapes from the soil it assists in soil aeration. Soil bacteria need carbohydrates in the soil; these are provided by organic matter. Soil-derived food must be in very dilute solution before it can be taken up by roots. In hot weather rapid evaporation occurs from leaves. Absorption of water by roots must be still faster, for this water contains additional food that will increase the sap concentration unless the newly absorbed solution is extremely weak. Over-concentration of cell-sap causes a de­mand for more water from the soil; if it not forthcoming, wilting results. If the concentration remains at its normal low level, growth is accelerated by heat, which, in this way, accelerates food intake.