122 



THE SHADING OP PLANTS 



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 



Fig. 177. Cheese-cloth shelter for vegetables. 



unquestionably 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 and 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. 



(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 soVeadily 

 and is more or less constantly in excellent workable 

 condition. 



Shading devices may have an important bearing 



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 te 

 undesirable in citrous 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- 



Fig. 178. Tents for growing tobacco. Conneoticnt valley. 



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 



