(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 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 essential to nitrogen assimilation. Just what in tensity of light may be the optimum for this par ticular function is not known, and there are doubt less complex relations to be considered. At any rate, the proteid content is usually less in shaded plants.
(6) It has been held that there is an increase in the acid content of shaded plants. This may be relative. A certain amount of acid lends quality and flavor, while an increase without gain in sugar may be deci dedly objectionable. In shading strawberries with cheese-cloth it has been shown that there is an actual reduction in the acid content. The acid ity, however, is more marked in taste, and this be cause of a marked reduction of sugar. The reduc tion of the sugar content, as well as of certain other carbohydrates in fruits, seems to be general under such cultural conditions.
(7) The aromatic products may not be very important as animal nutrients, but they are physiologically essential, and represent almost the sole value of eco nomic plants used as condiments. In 1838, De Candolle called attention to the diminished production of savors and odors in shaded plants. It was found later that plants removed from south ern latitudes to the latitude of Scandi navia during the two months of maxi mum sunshine in the latter region, showed an increase in the development of aromatic products. Indeed, it has long been suggested that many fruit-bearing plants containing objectionable flavors might be benefited by etiolation.
(8) In total darkness very few plants will develop normal flowers or fruit, even when grown from bulbs or other storage organs, and a general effect of etiolation is usually apparent in the reduction of fruit ing, while increased or continuous illumina tion often hastens flowering or fruitage, or may lengthen the flowering period. How
ever, when there is only partial shading it is quite possible that the size of succulent fruits may be increased, and the time of ripening hastened, for the moisture and temperature factors under half-shade will play important roles. It has been found, for instance, that under cheese-cloth sev eral varieties of strawberries bear a larger fruit ; and that lettuce runs earlier to seed.
(b) Indirect through the environment. —The practice of shading may modify the factors of the environment in a variety of ways ; and each of these factors is impor tant in the life relations of the plant. The purpose, of course, is primarily the modified light effect, yet frequently the effect on other factors is much more important. Aside from reducing the light, shading is important in the relations of the plant in order (1) To regulate hu midity.
(2) To conserve soil water.
(3) To mitigate or equalize temper ature.
(4) To give partial protection from wind.
(5) To mair.tain bet ter physical con dition of the soil.
(1) In wet periods, shaded plants may have no advantages, certainly none so far as the humidity is concerned ; but in dry weather the humidity is reported as more reg ular under partial shade. This relation is important in dry regions. It is a mistake to assume that because of greater humidity plants will always be more subject to fungous diseases. The relation of plants to fungous diseases is complex, and the general vigor of the plant is usually of more importance than any single environmental factor.
(2) The evaporation of water from the soil is unquestionabh• less under the covers used in shad ing, and this has been experimentally demonstrated time and again. The extent of the benefit would necessarily be determined by the dryness of the season or the region.
(3) Extremes of temperature are somewhat miti gated by shading. Radiation from the soil is pre vented to a considerable extent, and the light that does enter carries with it heat, much of which is absorbed. The minimum temperature under cover devices will always lag behind the minimum of the external air. Experiments in the North in early summer in moist seasons have shown a desirable in crease in the temperature under cloth cover. Other experiments in July and August, when the amount of sunshine is much greater, have shown a slightly lessened temperature under cover, yet a greater uni formity. Repeated experiments in the South, how ever, show that by shading a very desirable equali zation of temperatures is effected. In the famous market-garden add floricultural region of France, east of Toulon, many crops are grown throughout the winter under the protection of half-shade. The temperature thus secured is sufficient for the main tenance of growth in the semi-hardy flowers and vegetables.