OF THE ATMOSPHERE OF HOT-HOUSES. 325 



maturity, constitute the most complicated and difficult part of 

 the gardener's art. There is some danger in laying down any 

 general rules on this subject, so much depends upon the pecu 

 liarities of the kind under cultivation, and the endless train of 

 considerations connected with the process of forcing. 



The following rules, however, may be safely stated-, as deserv- 

 ing especial attention from the gardener in charge of hot-houses : 



1. Moisture is most required in the atmosphere by plants 

 when they first begin to grow, and least when their periodical 

 growth is completed. 



2. The quantity of atmospheric moisture required by plants 

 is, ccBteris paribus, in inverse proportion to the distance from the 

 equator of the countries which they naturally inhabit. 



3. Plants with annual stems require more than those with 

 ligneous stems. 



4. The amomit of moisture in the air most suitable to plants 

 at rest, is in inverse proportion to the quantity of aqueous matter 

 they, at that time, contain. Hence the dryness required in the. 

 atmosphere, by succulent plants, when at rest. 



Moisture in the atmosphere, then, is absolutely necessarj'' to 

 all plants, when they are in a state of rapid growth, partly be- 

 cause it prevents the action of perspiration becoming too violent, 

 as it always does in a high and dry atmosphere, and partly 

 because, under such circumstances, a considerable quantity of 

 aqueous food is absorbed from the atmosphere, in addition to 

 that drawn from the soil by the roots. 



Excessive moisture is injurious to vegetables in winter, when 

 their digestive and decomposing powers are feeble, and evapora- 

 tion from the soil should rather be intercepted than otherwise, 

 except when the atmosphere is dried to an unhealthy degree, 

 by the use of fire heat. 



One of the causes of the Dutch method of winter-forcing is, 

 undoubtedly, their avoiding the necessity of winter ventilation, 

 by intercepting the excessive vapor that rises from the soil, and 

 would otherwise mix with the air. For this purpose they inter- 

 pose screens of oiled paper between the earth and the air of their 

 houses; and in their pits for vegetables, they cover the surface 

 of the ground with the same oiled paper, by which means vapor 



