78 



Plant Physiology 



The amount of the exudation may vary from a few 

 drops to several liters per day. Large quantities have 

 been reported for a few plants, especially tropical or sub- 

 tropical forms ; thus Humboldt reports for the American 

 aloe 7.5 liters per day, or about 1000 liters during the 

 entire period ; while if the observations of Semler are 

 taken, Caryota urens may produce 50 liters per day, the 

 maximum amount observed. Among agricultural plants 

 employed in demonstration work, the potato and tomato 

 are good for short observations, and the grape vine - 

 less subject to decay - - for more extended experiments. 

 Eckerson finds that among common greenhouse species, 

 Fuchsia speciosa and Begonia coccinea are especially favor- 

 able for quantity. The pressure under which the exuda- 

 tion is produced necessarily bears no relation to quantity 

 of exudate. The following table, taken from the data of 

 Eckerson, indicates what may be expected of satisfactory 

 material in experimental studies : - 



