of Durham^ Northumberland, and Newcastle iipon Tyne. 25 



by a law exactly the reverse of this which obtains in nature. 

 Here the heated particles being intercepted in their ascent, 

 and confined by the glass roof, the top of the house, as prac- 

 tical men know well to be the case, will always be warmest, 

 and the temperature will rapidly decrease towards the bottom, 

 and nearly in a ratio proportionate to the degree of heat main- 

 tained ; hence the necessity for a permanent source of heat at 

 the bottom, not to keep the root warmer than the rest of the 

 plant, but merely to obviate its being in a colder situation. A 

 mild bottom heat, accordingly, is always found in practice to 

 succeed best. For the same reason, unless the plants are kept 

 very near the glass, a great circulation of fresh air, and con- 

 sequent waste of heat, is generally found necessary ; as, unless 

 the heated air at the top were thus suffered to escape, the 

 leaves and extremities of the plants, being attracted by the 

 warmer medium above them, would grow towards it faster 

 than the lower parts could supply nourishment, and thus 

 would become what gardeners term drawn. The necessity 

 for change of air, except in reference to temperature and 

 moisture, cannot well be accounted for on any other principle, 

 as I believe it has been satisfactorily ascertained that vegetable 

 life does not destroy the vital properties of air in the manner 

 that animal life does, but that, although the air is much altered 

 by it at one period of the day, it is restored to its former state 

 in another, and, on the whole, no material change is per- 

 manently produced. Mr. Knight, the scientific president of 

 the London Horticultural Society, condemning the bark-bed, 

 except for the purpose of striking young plants, has had a 

 house constructed for the purpose of growing stove plants 

 without bottom heat, and from time to time has given a detail 

 of his proceedings and the results (which have generally 

 proved favourable) at the meetings of the Society. In one of 

 his papers he states, that the pine plants which stood on the 

 hottest part of the flues, immediately above where the fire 

 entered, grew stronger and more luxuriantly than the rest. 

 This is exactly as might have been expected, for the plants 

 standing above the source of heat would have the benefit of 

 first receiving the heated particles of air in their ascent, and 

 consequently would be in a situation more congenial to nature 

 than those in othei* parts of the stove, where their leaves 

 would be in a warmer stratum of air than their stems and 

 roots ; though this was also diminished as much as possible, 

 by always keeping the plants in contact with the glass, which 

 was effected by placing the pots on pedestals of loose bricks. 

 The dispensing of bottom heat is a subject peculiarly deserv- 

 ing the attention of the horticulturists of this part of the 



