84 AGITATION OF THE ATMOSPHERE, 



stems of transplanted trees firmly to stakes impedes the growth of that part 

 of their stems which is below the tie ; and that trees, after being fixed two 

 or three years in this way, have their stems rapidly increased in thickness 

 when set at liberty. De Candolle mentions a tree, which had been tied up 

 in such a manner that it could only move from north to south, which at last 

 formed a trunk the horizontal section of which was elliptic. The effect of 

 motion on plants generally, he considers to be increased evaporation, and a 

 more rapid movement of the descending sap. (Phys. Vegetale, t. iii. p. 1178.) 



264. By greatly increasing the perspiration of the leaves and other parts 

 of plants, wind renders them less watery ; and when this is not carried to an 

 injurious extent, plants are by that means rendered firmer, drier, and better 

 adapted for being packed and sent to a distance. Hence greenhouse plants 

 grown in pits, where the atmosphere is continually moist, are less adapted 

 for being sent to a distance than such as have been grown in open, airy green- 

 houses ; and such as have been grown in houses heated by brick flues, are 

 better than those which have been grown in houses heated by hot water. 

 In like manner trees grown in nurseries, situated on high dry situations, 

 exposed to the wind, must necessarily have their wood harder and better 

 ripened, than such as are grown in moist sheltered valleys. The uses of 

 wind in the open air may be reduced to that of drying surfaces, and that of 

 putting plants in motion ; and the evils attending wind result from these two 

 properties being carried to an excess. All the advantages to be obtained 

 from wind in the open air in horticulture are to be obtained by exposure ; 

 and all the disadvantages are to be counteracted by shelter. In plant struc- 

 tures the imitation of wind, by the agitation of the air, will have the same 

 effect as in the external atmosphere, but in a diminished degree. It is also 

 of use, by rendering air at a high temperature more agreeable to the human 

 feelings than when it is in a stagnant state ; though some (268) consider that 

 this is to be principally attributed to the air being saturated or nearly so 

 with moisture. 



265. Shelter^ as every gardener knows, is produced by means of walls, 

 hedges, plantations, and other screens, placed at right angles to the direction 

 of the wind ; but the force of the wind is most powerfully reduced, not by 

 opposing a strong barrier, such as a wall, but by an elastic, partially open, 

 body, such as a hedge or a thin plantation. The most effectual mode of 

 sheltering any territorial surface, whether level or hilly, is by scattering 

 over it single trees. In this way, a park or pleasure-ground in the most 

 exposed situation may be sheltered in every part of its surface. In this way 

 also an orchard or plantation of fruit-trees, the trees being equally distributed 

 over the ground, produces its own shelter ; but as a kitchen-garden, if planted 

 with standard fruit-trees so as to produce shelter, would be unfit for the 

 culture of culinary vegetables, the best mode of sheltering it is by crossing 

 it with walls and hedges at such distances as may produce the desired shelter 

 in the given situations. A very efficient shelter for culinary vegetables may 

 be produced by sticking in branches of young trees, four or five feet in length, 

 like pea-sticks, all over the surface on which they are grown ; or by inter- 

 secting the surface with lines of wicker-work hurdles, which could be put 

 down and taken away at pleasure. By throwing the compartments of a 

 kitchen-garden into squares of ten or twelve feet on the side, by wicker- 

 work hurdles, an effective shelter would be produced ; and by covering these 

 squares with netting, resting on the hurdles, a great deal of the heat radiating 

 from the ground would be returned to it. Hedges of thorn, hornbeam, or 



