(4) Shading devices are not wholly unimportant from a consideration of the wind relation. There is, in the first place, a lessening of the mechanical injuries, and, in the second place, the prevention of desiccation or excessive loss of water at times when the water content should be conserved.
(5) Under cover the soil does not bake so readily and is more or less constantly in excellent workable condition.
Shading devices may have an important on all the above environmental factors, but, of course, it would be absurd to use such devices merely for the regulation of some of these, such as the conservation of soil moisture or the mainten ance of a good physical condition of the soil.
(c) Kinds of plants to shade.—Shading is appli cable to celery, rhubarb and tobacco un der a variety of conditions, and may be employed for cauliflower, lettuce, aspara gus and probably some other crops,— these all being plants commonly culti vated throughout the country. It is par ticularly applicable in pineapple-culture in Florida, and it has been shown to le undesirable in citrons culture in the same state. In addition, shading must be prac ticed to a certain extent in the cultiva tion of those greenhouse or floricultural plants whose native habitats are beneath the shade of the forests in subtropical or tropical regions. Among such plants are some species of ferns, palms, selaginella, anthurium, caladium, certain orchids and many others. Indeed, in the case of orna mental plants, a knowledge of the habitat will generally indicate the procedure to be used in their propagation with refer ence to light. Moreover, in some cases it will be necessary in drier regions to pro pagate under half-shade plants whose native habi tats are more moist. Under the severe sunshine of the Sahara, shading is practiced on a large scale, for the garden cultures are beneath the palms of the oases. In other lands, tea may be grown in forest glades.
The screen mechanism.
Lath screen.—The materials to be used in the construction of the shading screens will depend on local conditions and prices. One of the earliest forms of screening was a lattice composed of sepa rate lath screens supported on scantling at the height desired. Such screens are still in use where tropical plants are being propagated. The lath screen is durable but, of course, is expensive in most regions.
A desirable lath shed for half-shade work, suit able in the cultivation of pineapples or tobacco, may be made as follows : Posts of 2 x 4-inch or 3 x 3-inch pine are placed nine or ten feet apart the short way and fourteen feet apart the long way. For solidity these may be set about one foot in the soil. Boards sixteen feet long are nailed across the long way and spliced at the posts, forming the joists of the structure.
Stringers, 1 x 3 inches, are then nailed across the boards, the stringers in turn supporting plastering laths, nailed about one inch apart. The shed may be of any height desired, but for ease in cultiva tion it should be at least seven feet high.
Cloth screen.— In recent times, the cheese-cloth screen has come into very general use in tent-mak ing on a large scale. The screens may be either open or closed at the sides, and the height will vary according to the crop and the cultivation to be given. Details of the cost of such screens per acre are available. When 2 x 4 scantlings are used for posts and good support is given overhead by means of scantling and stout wire, the materials and labor have been variously estimated at $300 to $350 per acre the first year. The lighter grades of cheese cloth, which are preferable for most cultures, can not well be used a second season ; nevertheless, the cost for the second and subsequent years will be materially lessened. Heavier grades of cloth may be used in some cases. Cloth is now manufactured in sixteen and one-half-foot breadths for this pur pose. "Domestic " is sometimes to be recommended; for small, more resistant covers, such as for cold frames, this material may be treated with linseed oil.
A good shelter-tent for tobacco, and, conse quently, one suitable for almost any shade - crop, may be constructed as follows : Posts of pine, chestnut, locust or other durable wood, eleven feet long are placed two feet in the ground and sixteen and one-half feet apart each way. Sixteen - and -one-half-foot stringers, 2 x 4 inches, nailed at the top of the posts, run one way, and across the other way are stretched No. 9 cable wires, stapled to each post and secured at the bor ders of the field by stakes placed six to nine feet beyond the tent borders and connected by a base board. Two lines of smaller wire (No. 12) are placed between and parallel to the heavy cables, hence, five and one-half feet apart. The lighter wire may also be run along the stringers and baseboard, over which wire may be wrapped the selvage of the cloth when stapled. G. B. cloth of a special width (sixteen and one-half feet) may be employed in this construction, or a heavier grade if it is hoped to use the cloth through a second year. G. B. cloth is somewhat heavier than cheese-cloth. At the two open sides twelve-foot cloth may be employed. When the shade is desired for only a part of the growing season this construction may be consid erably simplified by reducing the height of the shed, the size of timbers, and the like.