LV ITS RELATION TO HORTICULTURE. 41 



pits, or other cool structures for the storage of half-hardy plants. In 

 all such structures it is more desirable to introduce dry air than 

 moist. Some of the great uses of fire heat in such structures is to cause 

 a motion or circulation of the air, to expel damp, and prevent the 

 growth of fungi, &c. 



Pits and cucumber frames, which are kept at a high temperature 

 during winter, frequently have the air within surcharged with moisture 

 to such a degree at that season as to endanger the health of the plants. 

 The ordinary remedy for this evil is to admit a portion of the external 

 air during bright sunshine ; but a safer mode, if it can be adopted, is 

 to admit the external air through tubes heated by being bedded in 

 dung or tan, or by being placed in contact with the flues or hot- water 

 pipes by which the pit is heated. By this means, the admitted air has 

 its capacity for moisture greatly increased, and it will absorb and 

 change the steam contained in the atmosphere of the pit, and the dew- 

 drops on the glass and framework, into elastic invisible vapour. 



In all plant structures change of air and ventilation are least neces- 

 sary when the plants are at rest, and most so when they are in full growth. 

 The more leaves there are upon a plant, or rather the wider the area 

 they occupy, the more air is required to enable them to perform their 

 respiratory functions. It is also found that increased ventilation and a 

 drier air are of great advantage to the maturation of the fruit ; but by 

 dry ness of the air must be here understood, not so much the 

 absence of invisible elastic vapour, as of steam, or watery exhalations 

 not held in a state of combination. " When grapes begin to colour, 

 it is of as much importance to obtain a dry atmosphere, as it was, pre- 

 viously, to have a moist one ; because the change effected in grapes 

 while ripening is produced under the full influence of light, heat, and 

 dryness : and it is well known that grapes grown in dry heat, in pro- 

 perly managed houses, acquire a flavour superior to those grown in 

 plant-houses where the air is kept moist for the sake of the plants." 



It is certain that, in all countries, the climate during the growing 

 season is moist, and at the ripening season comparatively dry ; and 

 hence the practice of withholding water from fruit-bearing plants under 

 glass, when the fruit is ripening, is in direct imitation of nature. It is 

 also natural to suppose, that in the ripening season in the open air, 

 when the surface of the soil is dry, the atmosphere over it will be 

 less saturated with vapour than when the soil is moist ; and, hence, 

 the recommendation of dry air for the maturation of fruits is also 

 natural. The effect of this air must be greatly to increase the perspira- 

 tion of the leaves, which is probably auxiliary to the increased action 

 of solar light, in the production of the saccharine matter and the pecu- 

 liar odoriferous properties of fruits. 



While, therefore, a free circulation of air is desirable at all times, 

 it is essential to the ripening of wood and finishing of fruit. Cold 

 draughts must, however, be avoided. Indeed, cultivators may lay it 

 down as a general principle, that neither water nor air ought to be 

 D to plants at a much lower temperature than that of the soil in 

 which they grow, or the air by which they are surrounded. 



