Transpiration and Water Movement 111 



employed (or of average leaves) upon paper of uniform thick- 

 ness, these outlines being subsequently cut out and weighed 

 accurately for comparison with the weight of a known area. 

 Prints may be made upon sensitized paper, or the planimeter 

 may be employed. The area of stems and petioles is generally 

 negligible, but may be roughly estimated when necessary. It is 

 well to express all transpiration data, as suggested by Ganong, 

 in grams per hour per square meter of surface, written g m 2 h. 



Amount of transpiration, determined by potometers. Set up a 

 transpiration experiment, employing either the Ganong potom- 

 eter (Fig. 24), a burette potometer (Fig. 23), or some other 

 form equally satisfactory. The former may be employed for 

 short periods, contrasting the effect of conditions ; while the 

 short burette potometer may be used for longer intervals, care 

 being taken, however, to keep the column of water in the burette 

 at a height approximately equal to that in the other arm. 1 



Effects of conditions upon transpiration. It has been indi- 

 cated that temperature, humidity, air movement, etc., are 

 directly and indirectly important in varying transpiration quan- 

 tities. While the effect of light variation may be demonstrated, 

 more satisfactory experiments may be made with the other 

 factors. 



A rough idea of the effects of temperature and humidity may 

 be obtained by simple transpiration experiments with simul- 

 taneous observations upon simple thermometers and hygrom- 

 eters, placing plants of more or less equivalent areas, even for 

 a few hours, under conditions determined to be diverse. With 

 more or less equal lighting, contrast the transpiration, for ex- 

 ample, in a moist basement room with that in a warm upper 

 room ; or, at a uniform temperature, also, contrast a plant 

 exposed in a quiet room with one in the same room under a large 

 bell glass, the latter securing greater humidity. Diverse con- 



1 In all transpiration or other experiments where the further ab- 

 sorption of water by excised shoots is required, the cutting of the shoot 

 should be done while it is bent under water, and the ends of the shoots 

 should be promptly immersed in water until used. 



