226 VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 



greater amount of cold than they could otherwise have endured, 

 and this, too, apart from the application of artificial heat. When 

 half-hardy plants are destroyed by frost, its effects are most 

 frequently visible at the collar, or lower part of the stem, arising 

 from the intense action of the cold at the surface of the ground, 

 which, in combination with moisture, first contracts and then 

 expands the principal sap-vessels of the plants. 



The annexed cut represents a double range of plant-pits, 

 heated by wooden tanks. These tanks are supplied from a 

 small boiler, placed in the centre, between the two pits ; #, end 

 section, shows the end of the tank, which is about six inches 

 deep, and divided into two compartments, by placing a slip of 

 wood up the centre, leaving a space at each end, for the water 

 to circulate round. The arrows show the course of the water 

 in its progress round the tank ; the flow and return pipes are 

 represented by dotted lines. These tanks are merely shallow 

 boxes of wood, occupying nearly the whole inner area of the 

 pits, and resting on piers of brick, or posts of wood ; rough 

 pieces of wood are laid crossways over the tanks, and a layer 

 of broken bricks, (or sawdust, if the pots are to be plunged, 

 which is desirable,) which forms the bottom, or floor, of the pit. 



It is truly surprising how very little fire is required to main- 

 tain a perceptible warmth in these pits ; and the growth of plants 

 or vegetables of any description is astonishing. In some nurs- 

 eries these pits are kept continually at work. The lights are 

 entirely thrown off them, and the tops thoroughly exposed to 

 the air ; this prevents them from being drawn up tender and 

 etiolated, and while their roots are stimulated with an agreeable 

 warmth, they have, nevertheless, all the strength and hardiness 

 of plants grown in the open air. 



For the growth of early melons and cucumbers these pits are 

 admirably adapted ; they are equally efficient, without having 

 the disadvantages of dung-beds. Their neat and tidy appearance 

 gives them a place beside the other hot-houses, (which is not 

 the case with hot-beds of manure,) to none of which they 

 yield, in point of utility or interest. 



If there is any one branch of exotic horticulture that possesses 

 more extended interest than another, it is, undoubtedly, the cul- 



