THE SHADING OF PLANTS By B. M. Duggar The shading of plants is a relative expression. It is qualitative and means simply reduced light intensity. As used by horticulturists, shading has reference most frequently to half or partial shade, or to the growth of plants under some form of improvised screen. The extremes of shading are great ; and properly to circumscribe the subject we must consider all plants exposed to grades of a light intensity between bright diffused light, as one extreme, and the darkness of cellars and caves, as thu other extreme.
Shading is a distinct phase of horticultural work, and it has its physiological, or fundamental, side. Such quantitative physiological work as has been done relates, for the most part, to absolute shading, or darkness, and an insignificant amount of accurate physiological data have to do with half or partial shade, which latter is more important horticulturally. The physiological work is not yet so halpful as it might be, but some of the general principles modifying form, size and quality of plants in shade or darkness enable us better to direct half-shade operations, and better to interpret the results that may be secured. The subject offers an interesting field of investigation.
The plant.
(a) Direct effect.—It must be borne in mind that plants are very differently adapted to light in tensities. Some plants to a large degree are in dependent of light conditions. Certain small fungi grow equally well in total darkness or in strong diffused light. The common mushroom, so far as the production of the fruit, or mushroom proper, is concerned, is uninfluenced by light, ex cept in so far as light affects temperature and, thereby, evaporation. Among common green plants there are shade-loving and sun-loving species. In the shade of certain trees, no green plant may live constantly. In the deepest gorge the densest ferns may grow, and on the exposed cliff a grass or a heavy vine may find its suitable home.
In considering the direct physiological effects of shading on plants, we may note the effect on (1) Color: Etiolation or blanching.
(2) The form and size of the plant.
(3) The minute structure, i. e., on the elements of the framework which have to do with texture and suc culence.
(4) The bulk of the plant, by reducing or modifyingthe products of growth.
(5) The checking of nitrogen assimila tion and albuminoidal synthesis.
(6) Modification of the acid content, as well as the content of soluble carbohydrates.
(7) The aromatic content in the plant juices, and other minor meta bolic modifications affecting the quality of the product.
(8) The development of flowers, fruits and seeds.
(1) The effect on color is considerable. The intensity of the light will usually directly affect the chlorophyll development. In darkness most plants are soon etiolated, or blanched, and many are much affected in half-shade. The produc tion of brilliant color is also less under dimin ished light. In garden products blanching may add (Erectly only to the appearance or tenderness, freshness or crispness; it is in its indirect relation to other modifications discussed below that it is most important.
(2) The ordinary green plant shows, with the exclusion of light, either partial or complete, an elongation of the main axis accompanied by some suppression of branches. This is of little practi cal advantage. Plants with restricted stems, and consequently with basal or truly "radical" leaves, usually show an elongation of the petiole with re duction of the leaf-blade. This effect is of prac tical value when the plant has been grown pre viously under full light, and has accumulated in root and stem an abundant supply of nutriment. A crop in point is rhubarb when grown by the "new culture" method ; and celery is somewhat similarly influenced in addition to the blanching effect.
(3) The diminished development of tough fiber in etiolated plants has been known since the time of Sir Humphrey Davy, and even earlier. The re duction is largely in the amount of mechanical or supporting tissues. This effect is an advantage when succulence is a chief concern. It is true of the crops mentioned in the preceding para graph, and it may also be of interest in growing certain salads.