26 THE UNHEATED GREENHOUSE 



merely but a system of ventilation. Free circulation of air is 

 indispensable, but it is not always recognised on the other 

 hand that draught is most injurious to plant growth. The 

 shrivelling of tender foliage, especially of Fern fronds, 

 generally set down to want of water, is due, more often than 

 is suspected, to exposure for a few hours to cross-currents 

 of air. It should be possible, therefore, in all greenhouses to 

 "put on " and " take off" air, in gardening phrase, according 

 to the way of the wind, and in order that this may be 

 done effectively ventilators fitting closely and firmly must be 

 provided both at the top and at the sides of a plant-house. 

 A span roof 20 ft. in length should have four ventilators 

 placed alternately at each side, and over these, upon the 

 ridge, it is well to have a wooden cap as an additional safe- 

 guard against drip. Strong hinges of the kind called " water 

 joints " are to be recommended, as being more durable than 

 the ordinary make. In a long house it is a great saving of 

 labour to be able to open and shut these top ventilators 

 simultaneously by an iron winch. Side ventilation is provided 

 for, either by having the lights made to open or by wooden 

 shutters let into the brick walls. The latter alternative is in use, 

 mainly, when the span roof rests upon the wall- plate a form 

 of glass-house rarely seen in any but professional gardens, 

 though such " pits " are admirably adapted for plant culture. 



It is difficult to lay down precise rules for ventilation, 

 but as a general principle it must be understood that the 

 treatment of plants in an unheated greenhouse is in strong 

 contrast to that given in the stove, where a close humid 

 atmosphere does no harm. There are not many days in the 

 year, even in heavy fog, when at least a chink may not be 

 left open to allow free circulation of air, for though it is a 

 common practice to keep greenhouses closely shut in foggy 

 weather, a little dampness from outside is actually less harmful 

 than stagnant damp within. In winter it is always safer 



