74 TRUCK-FARMING AT THE SOUTH. 



Bodies thus exposed must radiate as much heat during 

 a wind as in a calm, but in the former case the con- 

 stant contact of warm air will return to them nearly as 

 much heat by conduction, as they lose by radiation, and 

 only a slight agitation of the air is sufficient to thus pre- 

 vent dew and frost. The reason why depressed locations 

 suffer most from cold by radiation is, that they are more 

 becalmed; and there must also be less dew in them, because 

 of the calm atmosphere which provides the moisture. It 

 is true that at considerable altitudes the air becomes colder 

 as the hight increases, but on hills, in cold and clear 

 nights, the frosts are less severe in consequence of the 

 movement in the atmosphere. The dew, which has been 

 deposited upon vegetation by condensation from the at- 

 mosphere, will become hoar or white frost, when the 

 object upon which it has settled is cooled by sufficient 

 radiation to congeal the water into crystals of ice. This 

 can only occur in this latitude up to 44 F. of the sur- 

 rounding atmosphere; or in other words, frost is impossi- 

 ble, unless the thermometer falls as low as 44. 



The different effects of the several solar rays are yet 

 imperfectly understood; but there is no doubt that the 

 heating and illuminating rays produce different results. 

 Plants supplied with heat and moisture may grow for a 

 short time in darkness; but there will be no development 

 of chlorophyll, or leaf -green, and they cannot thrive. 

 Plants, in all stages of growth, need the presence of at- 

 mospheric air, from the seed requiring oxygen for its 

 germination, to the plant which acquires its chief supply 

 of carbon from the air. Water is absolutely necessary in 

 the economy of vegetation. 



The management of plants under glass, whether they 

 are to be transferred to the open ground or not, requires 

 an acquaintance with the effects of these various agents 

 and phenomena, so that they may be made to harmonize 

 in the production of a sturdy and healthy vegetation. 



