22 PLANT PHYSIOLOGY 



too variable both in themselves and in their relations to external 

 conditions. Nevertheless, some form of expression of physi- 

 ological quantities is certainly desirable if not essential, in order 

 to give definition to knowledge of the respective subjects. Thus 

 it is vastly better, for example, to know that green leaves in 

 bright light in summer form about one gram of food substance 

 (photosynthate) per square meter of leaf per hour, generalized 

 and inaccurate for most plants though that statement is, than 

 not to know anything at all definite about the amount. Accord- 

 ingly in my own teaching, and in this book, I have determined 

 for each process the mean of all the available quantities, have 

 expressed this mean in the nearest round number (taking it in 

 the direction of greatest probability or frequency), and have 

 adopted this number as the constant of the process, giving warn- 

 ing of its nature, however, by calling it the conventional constant. 

 In some cases it is desirable to know not only the mean but 

 also the extremes of a process, in which case, whether expressing 

 the exact quantities or their round numbers, I have linked the 

 three together by hyphens with the mean in the middle in heavier 

 type. Furthermore it is also desirable, both for convenience 

 of comparison and also as an aid to the memory, to state all 

 constants in as nearly as possible the same system of units, 

 which should be capable of clear, brief expression. Accord- 

 ingly I have adopted the standard of grams per square meter of 

 area, or per kilogram of weight, or per liter of bulk, per hour, 

 always stated in this order and expressed in brief as gm 2 h, gkh, 

 or glh respectively. The large units of meter, kilogram, and 

 liter have the advantage of permitting most physiological quan- 

 tities to be expressed in whole numbers, with a minimum of 

 decimals. These conventional constants are not intended for 

 investigation purposes, though some such use they may inciden- 

 tally have; but they are provided for the education of the stu- 

 dent, who should learn them by heart and have them ever ready 

 for use. They cannot be misleading if their conventional char- 

 acter is kept in mind. A synoptical Table of those already 

 worked out will be found in Part III. 



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