28 PLANT PHYSIOLOGY. 



opaque white to reflect as much light and heat as possible so 

 the dark-room will not become too much heated. The outer 

 roof of the dark-room, placed just below the greenhouse roof, 

 is of matched boards painted white above and black below and 

 with a special projection in front as shown in Fig. 4. Beneath 

 this comes a double roof all painted black, arranged as shown 

 in the figure, to allow the escape of air, forming a part of a 

 ventilation system. Entrance for the air is arranged thus: in 

 the row of bricks just above the bottom row on the three thin 

 walls, each alternate brick is to be omitted, and a wooden box, 

 painted black, in the form shown in section in Fig. 4, is to be 

 built all around the three sides (except of course where the doors 

 are). This arrangement will admit air freely but no light, and 

 the air will escape without admitting light through the double 

 roof above. A small dark-room, or rather box, on this prin- 

 ciple has worked well through two years' use. The brick outer 

 walls may be painted white ; and shelves are to be placed at 

 convenient heights on the inner walls. The floor is of cement. 

 The heating-pipes do not run into the room, as the air-space 

 extending practically all around will make it hold approxi- 

 mately the temperature of the greenhouse. 



The shading of every greenhouse from the too intense sun 

 of some days is important. This can be effected by the usual 

 whitening of the glass in the spring, but for physiological pur- 

 poses it is much better to have a system by which the shading 

 is not only removable at will, but is alterable in different 

 degrees. This can be effected, though with some trouble, by 

 having lengths of cheese-cloth which can be pinned to the 

 beams of the greenhouse roof by spring clothes-pins. What 

 appears to be a better system is to be tried in our new green- 

 house. Stout galvanized-wire frames, 8 feet long and 6 inches 

 wide, are to be covered with thick cheese-cloth and hinged by 

 one edge to the greenhouse roof, stretching across between the 

 beams. A cord is to connect the middles of the lower edges, 

 and by drawing on this cord the frames can be set parallel with 

 the sun's rays, when they will give practically no shade at all. 



