BOTANICAL STRUCTURES. 



283 



glass. The main part of the area is occupied by a stage 

 rising in steps to receive the potted plants. At some 

 height above the front flue is placed a narrow horizontal 

 bench of trellis work, to receive pots containing small 

 plants which require to be near the light. In England, 

 since the repeal of the duty on slate, this material has 

 been in many cases advantageously employed in forming 

 the pavement, the shelving, and stages of plant-houses. 

 The interior air is warmed by one or two flues, or other 

 heating apparatus, according to its volume. If a temper 

 ature of 45 Q Fahr. be maintained during winter, it is suffi 

 cient. Sometimes green-houses are constructed with span- 

 Fig. 18. 



