Moisture Plants growing in soil and air uniformly moist have smaller root systems and a larger total leaf surface than they would have if grown under constantly varying conditions of soil and atmospheric moisture. A plant reared in a moist greenhouse will gradually shed its older leaves if the air is allowed to become too dry or the plant is removed to an ordinary dwelling-room. The new leaves that develop under such circum stances are adapted to the drier air and are better able to retain the water furnished by the roots. If, however, the change from moist to dry is too sudden, the defoliation will be exces sive and the new growth may develop very slowly and be poorly nourished. In the case of house plants there is almost a universal tendency to over-water the plant at such times. This brings on suffocation and death of the root hairs and feeding roots, and ends in death of the plant. Instead of watering under such conditions the soil should be allowed to become comparatively dry until renewed growth of leaves create a demand for water from the roots.
Plants which have been growing for a con siderable period with a comparatively small water supply develop a large number of root hairs and firmer and smaller leaves adapted to reduce transpiration to its lowest terms. If a cloudy, wet period suddenly follows, or the plants are over-watered, they become gorged with water. The more plastic tissues start into growth, with the result that the leaves often become distorted, wrinkled, curled or twisted. If there is a continued excess of moisture many of the root hairs and finer roots may die, and symptoms of root suffocation develop, the leaves become spotted with yellow and finally drop off.
CEctema is a pathological condition charac terized by the formation of water blisters or warts on the stems or along the vascular bundles of leaves over-gorged with water. This dis eased condition often develops in plants growing in moist situations, especially under conditions where the soil is likely to be wanner than the air, thus favoring absorption of water by the roots, but not favoring transpiration through the leaves. The disease often develops, also, in extended periods of moist, warm, cloudy weather.
Burning or The growth that plants make under cloudy, moist conditions is often watery and soft, an easy prey for parasitic fungi and likely to dry out, wilt and die easily if suddenly exposed to clear, hot weather. Such °burning)) or ((scalding)) of the edges and tips of leaves is very common in the spring, when moist, cloudy periods are suddenly followed by clear, hot days.
Wind often causes injury by whipping and tearing the leaves or breaking limbs. In the latter case injuries are produced which predis pose the plant (especially trees and shrubs) to attack by insects or fungi. Wherever possible
all such injured parts should be cut out to unin jured tissue and protected by antiseptic washes or grafting wax. Dry, hot winds coming early in the summer often cause the death of tender foliage by desiccation.
Injurious Slight traces of illumi nating gas escaping in rooms or greenhouses, or the slightest trace of sulphurous fumes from burning coal from furnaces or stoves, is ex tremely injurious to vegetation. The presence of these gases in the atmosphere causes the gradual yellowing, browning and finally the death of leaves, especially the tips and edges. Although the soil, temperature and light condi tions may be the most favorable, the plants gradually die where they are more or less con stantly exposed to a trace of these gases. In the neighborhood of factories or large cities vegetation is often injured, as described, by these and other poisonous gases. In some cases the amount of poisonous gas thrown into the air with the smoke from certain factories is so great that vegetation of all kinds is unable to thrive in the neighborhood. The injury ex tends often several miles from the factory in the direction of the prevailing wind.
Many predispositions to disease, as well as specific pathological condi tions, are produced in plants as the result of un favorable temperature conditions. As a rule, whets growth is taking place and the cells con tain •much water, vegetation is most sensitive to heat and cold. When growth has ceased and the tisses are mature or in a resting or dormant condition and contain much less water, they are least sensitive to heat and cold. The tempera ture range for any individual during growth, and even in the dormant or resting condition, varies for different individuals, varieties and species. These temperature ranges have be come more or less fixed within certain limits for different individuals and species through the in fluence of the temperature factor in the environ ment under which the species or individuals have developed. Thus, plants adapted to long growing seasons of the South will not thrive in the shorter, colder season of the North. On the other hand, many plants adapted to Northern conditions do not thrive in the South. When an individual or a species becomes adjusted or accustomed to the conditions and changes of a particular environment it may be unable wholly or in part to adjust its life processes to an un usual change in any factor or group of factors or the relation of these to each other. The orange, lemon, lime and numerous other sub tropical fruits, for example, do not have a well defined resting or dormant period like the apple, pear, peach, etc., in the North. They would, therefore, be quickly killed if exposed to severe frosts.