Studies on phloem exudation from Yucca flaccida Haw. VIII. Fluid mechanics and exudation

Calculations on the linear velocity of the flow of exudate in sieve tubes were carried out, using measured average values for the length and cross-section of the sieve tubes, the surface area and the thickness of the sieve plate, and the number and radius of the sieve pores. Assuming sieve pores free of any obstruction a velocity of flow of 128 cm.hr-1. atm-1 through one metre of sieve tube was found to be theoretically possible. With a pressure difference between the intact sieve-tube members and the exuding ones of at least 16 atm – at an assumed distance of one metre, the fluid mechanical capacity of the sieve pores of the Yucca inflorescence would consequently allow an exudate velocity of 20 metres, hr-1. From the rate of bleeding a velocity of the exudate in the sieve tubes of 0,44 m.hr-1 was calculated, thus only a small part of the pores need to be unobstructed during bleeding. In a previous study sieve pores appeared to be often traversed by 10-20 protoplasmic filaments of approximately 250 A diameter. Assuming all the sieve pores obstructed by such filaments, spaced at the observed distance of 1000 A, calculations indicated that a pressure difference of 51 atm.metre-1 would be sufficient to explain the earlier observed velocity of exudation. It is concluded that the hypothesis of a mass flow of assimilates from the leaves to the top of the inflorescence of a bleeding plant does not encounter serious objections from the side of fluid mechanics.