FURTHER STUDIES ON EXUDATION IN CUCURBITS

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Abstract
Phloem exudate was collected from squash plants for periods of 12 and 24 hrs. Exudation fluctuated in rate but did not stop. Exudation from excised stems depended in volume upon the size of the stems and not upon the condition of local tissues. Excised root systems of squash exuded more than their own weight in xylem sap in 24 hrs. Phloem exudate is 8-20 times as concentrated as xylem exudate. Phloem sap from cucurbits coagulates with heat, alcohol, or long standing. Xylem sap does not coagulate. Phloem sap exudes in relatively small quantities and soon stops. Exudation can be maintained only by frequent cutting. Xylem sap exudes in much larger quantities and flows for hours without interruption. Phloem exudation will take place from wilted plants. Xylem exudation occurs only where transpiration loss has been greatly reduced or eliminated. Phloem sap had a pH of 7.3 and was well buffered. Xylem sap was practically unbuffered and had a pH of 5.5. Plugging of the phloem apparently results from the coagulation of the phloem sap within the interstices of the walls of the cut phloem elements.