120 



THE SHADING OP PLANTS 



A general discussion of shading involves the 

 following considerations: 



I. The Plant. 



(a) Direct effect on the plant. 



(6) Indirect effect on the plant 



through environment. 

 (c) Kinds of plants with which the 



operation of shading may 



be employed. 



II. The Screen Mechanism. 



Laths and boards, cloth screens, 

 plant covers. 



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 planbs, 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 



modif yingthe 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 directly 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. 



(4) The dry weight of certain shaded plants is 

 less than of plants under normal light intensity, 

 and this probably is due largely to the lessened 

 chlorophyll activity. In this connection, however, 

 it is important to remember the specific light re- 

 lations of the plant. It is asserted that under 

 favorable conditions of temperature and moisture 

 the common evening primrose {CEnothera biennis), 

 a sun-plant, has the power to fix in direct sunlight 



173. A tea nursery in South Carolina. (C. TJ. Shepard.) 



about three times as much CO2 as in ordinary dif- 

 fused light. The common polypody (Polypodium 

 vulgare), on the other hand, has shown a more 

 energetic assimilative (photosynthetic) activity 

 in diffused light than in direct sunlight. This 

 doubtless would be true for the ginseng. Indeed, 

 it may be said that shading is an antidote for ills 

 with one species, while with another it may prove 

 a bane. Varieties may likewise show diverse sun 

 relations. It is therefore of comparatively little 

 value to make shading tests with only two or three 

 of many diverse varieties of a cultivated plant, 

 the extremes of whose light relations have been 

 merely assumed. 



(5) In ordinary green plants light seems to be 



