390 PROTECTION FROM ATMOSPHERICAL INJURIES. 



to a variety of contrivances to protect them from atmospherical injuries. 

 The most effective of these is without doubt that of forming for such 

 plants artificial climates, such as the different kinds of hot-beds and hot- 

 houses; but there are also various contrivances for protecting plants growing 

 in the open air or against walls, and it is to these that we at present intend 

 to confine our attention. They may be included under shading from the 

 sun, sheltering from wind, and protecting from rain or from cold. Most of 

 these operations are founded on the doctrine of radiation, which has been 

 treated in so much detail in chap, iv., p. 67, that very little more requires 

 to be said on the subject. 



835. The object of shading is to lessen evaporation from the soil or from 

 plants, or to exclude light or heat. It is effected by interposing some opaque 

 medium, or even glass in some cases, the purest of which as we have seen 

 (486) excludes a certain portion of light, between the objects to be shaded 

 and the direct rays of the sun, and this medium differs in its texture and 

 other properties according as it is intended to be temporary or permanent. 

 Mats and canvas are the common articles for temporary shading in the 

 case of plants under glass ; but for plants in the open garden, hurdles of 

 wicker-work, or frames filled in with beech or birch branches, screens of 

 reeds are used, or the plants are placed in the north, east, or west sides of 

 walls or hedges. Sometimes also they are planted under trees ; but as this 

 kind of shade excludes rain and dew, it is only adopted in particular cases. 

 A slight degree of shade is produced by forming the surface of ground into 

 narrow ridges in the direction of east and west, and sowing or planting the 

 crop on the north side of the ridge. On the same principle, crops in rows 

 in an advanced state are made to shade seedling crops sown between them, 

 when shading them is desirable. Oil paper-caps, and other articles for 

 shading individual plants have been figured and described (449), and also 

 canvas shades for hot-house roofs (464). Some of the most severe injuries 

 which plants trained against walls sustain in this country is by the powerful 

 action of the sun in early spring, succeeded by extreme cold ; but by judi- 

 cious shading such evils may be greatly mitigated or altogether avoided. 



836. Sheltering from wind, the principles of which have been pointed out 

 (265), is effected on a large scale by plantations, and in gardens by walls, 

 hurdles, wicker-work covers (461), hand-glasses, and other articles described 

 and figured in sect, vi., p. 158. 



837. The principles of protecting from cold have been described at length 

 in our chapter on the atmosphere (p. 67), and the different materials and 

 contrivances for this purpose have been enumerated in the section (p. 158) 

 just referred to. Coverings for the surface of the ground include dead 

 leaves, litter, straw, sawdust, spent tan, rotten dung, coal ashes, coarse sand, 

 spray, and branches of trees or shrubs, &c. Coverings for standard plants 

 in the open garden include temporary roofs of thatch, boards, canvas, wicker- 

 work, bark, or manufactured materials, such as pitched paper, asphalte 

 sheeting, &c. Coverings for walls include branches with the leaves on, 

 such as those of the silver or spruce-fir, of the beech, birch, or hornbeam, 

 cut before the wood is ripened, in consequence of which the leaves will 

 adhere to the shoots, and being dead and without moisture, they are better 

 non-conductors than green leaves, straw or hay ropes, rope-netting, canvas, 

 bunting, woollen-netting, oiled paper-frames, wicker-work, hurdles, &c. 

 By referring to p. 173, it will be seen that thin canvas has been found the 



