PLANT PHYSIOLOGY. 



reduces it to millimeters of mercury) as if no vapor-tension were present, 

 and then adds to this result the pressure obtained for the given temper- 

 ature from the table, either reduced to fractions of an atmosphere or in 

 millimeters of mercury. 



It is usually not difficult to bring the gauge for the final reading into 

 approximately the same temperature as that at which the first was made; 

 but if this cannot be accomplished it is easy to make the correction for 

 it by the calculation that the gas in the tube expands or contracts ^3- of 

 its volume for each degree of temperature C. 



In experiments involving pressures, wherever there is an outside free 

 surface of mercury or water, a correction should be applied for baromet- 

 ric changes, but here again the possible error is hardly great enough to 

 materially affect the comparatively gross measurements in these experi- 

 ments. But the fact that all these errors exist, their nature, and the 

 method of correction should be illustrated. 



3 (page 69). Not only with these, but with all gauges, it is neces- 

 sary to insure that the gauge can register a maximum pressure within 

 the amount of movement which the plant has available for showing 

 the pressure. Thus, if water is being given off from the plant at a pres- 

 sure of one atmosphere, but the quantity is only 20 cc., and a compara- 

 tively large open gauge is used, obviously the 20 cc. of water would not 

 be sufficient in quantity to push the mercury 760 mm. high, as it must to 

 register the pressure of an atmosphere. Hence the gauges should be of 

 such a sort that several atmospheres can be registered by a much smaller 

 quantity than 20 cc. This applies also to the use of spring-balances for 

 registering pressures of swelling seeds, etc. 



4 (page 79). After somewhat extended quantitative tests of various 

 methods of enclosing the soil to prevent evaporation, I have concluded 

 that the two methods possessing the greatest advantages are the follow- 

 ing: A zinc pot is made of a size to cover the flower-pot; it costs but 

 little and can be used for years. The top is strengthened by a wire, just 

 below which a groove is made. The rubber sheeting, with a hole in its 

 center, made by a large cork-borer, is then used as previously described, 

 to connect plant and zinc-pot, and it is fastened to the latter by wire 



