ON GREENHOUSE PLANTS. 



701 



When there are only three or four houses and frames one 

 boiler will be sufficient, its size varying according to the area 

 to be heated. In this case, when there are only one show 

 house, two growing pits, and some frames, there should be 

 separate flow and return pipes for each, the heat being regulated 

 by means of valves. The boiler must be below the level of the 

 houses, and have a feeding cistern a little higher than the 

 highest pipes, which must always be kept full of water. In the 

 growing pits the warm division will be at the end by which the 

 pipes enter — next to the boiler; they will then pass on into the 

 cool end, being regulated by valves in both, so that if desired the 

 heat may be cut off from the cool end whilst it is still retained 

 in the warm one. 



Four-inch pipes should be used, and plenty of them ; it is 

 far better to have a gentle uniform heat from a number of 

 pipes than a fierce, scorching heat from a few, which is very 

 injurious, making it necessary to drive the fire much harder, use 

 more fuel, and thus wear out the boiler quickly. The pipes 

 should pass along under the stages all round the sides of the 

 houses. It is also a good plan to carry two " flows " and " returns " 

 along the roof to dry up excessive moisture and prevent drip,, 

 and in the warm pit to have a couple along the top of the shelf 

 next the outside wall. 



In summer no artificial heat will be required for any of the 

 houses except in very damp or cold weather. 



Ventilation. — ^The art of giving the right amount of air at 

 the right time can only be acquired after long experience. All 

 the houses should be plentifully supplied with ventilators,, 

 especially the greenhouse. This should have lights on hinges 

 along each side of the house and along the whole length of the 

 lantern on either side, and wooden ventilators under the shelves,, 

 let in the brickwork on a level with the pipes, all opening from 

 the inside. 



Plenty of air should be given on every occasion when it can 

 be done without lowering the temperature unduly. Fresh air is 

 life to plants, but when cold and damp it is injurious, and 

 should be excluded. When a cold or strong wind is blowing 

 only those ventilators on the opposite side from that which is 

 exposed thereto should be opened. Bottom air can always be 

 given by means of the ventilators in the brickwork, except when 

 the thermometer stands below freezing-point or the wind is very 

 cold. By this means the air of the house is kept fresh, whilst by 

 passing over the pipes first it is warmed before reaching the plants. 



The same rules apply to the growing pits, except that not 

 quite so much air should be given to many plants making 

 growth as to those that have finished flowering or are in full 

 blossom, and this shows the necessity of having a warm as well 

 as a cool pit even in summer. Each pit should have ventilators 



