256 BOTANICAL GAZETTE [april 



in the accompanying fig. i. By this apparatus the time and amount 

 of transpiration are recorded continuously. 



Each plant was under observation for six consecutive days (Mon- 

 day morning to Saturday afternoon) : climatic changes were recorded 

 in the near vicinity by the thermograph, hygrograph, and barograph, 

 but as changes of pressure have no appreciable effect on the transpira- 

 tion the records are not given. There is no satisfactory instrument 

 recording the varying intensities of Hght and darkness in a form per- 

 mitting the construction of a graph. Therefore the changes of light 

 were recorded from observation and were afterward reduced to a 

 graph form by the following method.^ Clements' table* of light 

 intensities for the different sun altitudes (the angle -of the sun's rays 

 with a horizontal surface) for Lincoln, Neb. (41° N. lat.), was taken 

 as a basis and changed to suit the latitude of Northampton (42° 19" N.) . 

 Then, since the intensities for any hour of the day obtained from 

 measurements on the celestial globe and graphically represented for a 

 briUiant cloudless day form a parabola with greatest value at sun 

 noon, it was possible to construct graphs showing full sunlight for 

 each day of the year. For cloudy days the percentage of intensities 



1 J • « JB — 



determined 



being those of full sunlight for the given time. 



Slight fleecy clouds and haze 90 per cent 



Heavy clouds alternating with blue sky 85 per cent 



Light open clouds, shadows cast 80 per cent 



Overcast, dark clouds, not rainy 60 per cent 



Overcast, with dark clouds and heavy rain 50 per cent 



Overcast, with dark clouds and snow 40 per cent. 



The second plant was intended partly as a check upon results 

 obtained from the first, but especially as a standard exposed to condi- 

 tions which admit of comparison of one kind with another, and which 

 can be repeated by other students. This plant was placed in a large 

 glass case {fig. 2) of some 652 liters capacity where conditions were 

 partly controlled, and were registered by the thermograph and hygro- 

 ^aph. ^ The temperature was kept within the range of 19-21° C. and 

 the moisture between 45 and 55 per cent, of saturation. The tempera- 

 tow ' Tl ^A '"^ *^^ ^^l^^lation of the light values I wish to thank Dr. Harriet Bige- 



^Clements, F. E., Research methods in ecology 57. 



1905 



