40 PLANT PHYSIOLOGY 



especially gas-fumes and escaping gas, must be kept from the 

 house, since, although some plants do not mind them, others 

 are thus very much, or even fatally, injured.* To prevent both 

 the drying out of the house, and also any possible entrance of 

 gas, the doors to the laboratory, and, especially, to the gas room, 

 should be kept always closed. No doubt in the former case a 

 tight-fitting door opening either way with a push would be best. 

 Equally important with the treatment of the house is the care 

 of the plants under experiment, which the student must himself 

 learn to attend to. Each plant should receive individual care 

 and attention, and should be kept clean, watered, and generally 

 well groomed. The golden rule of watering should be followed, 

 namely, to give a copious watering at intervals, allowing the 

 soil to partially dry out between times, rather than to give a little 

 water often, which prevents free aeration and respiration of 

 the roots. It is possible to tell whether or not plants are in need 

 of water by the gardeners' test of tapping the pot, a "ring" 

 meaning a need for more water, and a dull sound indicating 

 that it still has enough. Further, the plants should never in 

 winter be given water taken direct from the tap, for this chills 

 them; but in the house there should always be a large dish or 

 tub of water kept standing long enough to take the house tem- 

 perature, and from this the water should be dipped and applied 

 by a small watering-pot. It is sometimes a good plan, in order 

 to prevent too great drying out of a pot, to place the latter in 

 a larger pot with moist sphagnum moss between. Finally, as to 

 the appearance of the house, it should not be allowed to show 

 rust or neglect, but should be thoroughly scrubbed, the floor 

 often, and the walls and roof once a year, and it should be kept 

 painted white inside and out. This is not simply for its preser- 

 vation, but also for the physical comfort and the moral support 

 of the workers therein. 



For a thorough college course in Plant Physiology, some 

 form of greenhouse is essential, but a good deal can be done in 



♦ There is an experimental study of this subject by Richter in Berichte 

 der deutschen botanischen Gesellsthaft, 21, 1903, 180. 



