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, plagéd 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. Ifa temper- 
ature of 45° Fahr. be maintained during winter, it is suffi- 
cient, Sometimes green-houses are constructed with span- 
Fig. 18. 
