Abstract
The theory that mass flow of organic materials in solution occurs through sieve tubes, and is facilitated by perforation of the sieve plates, is open to criticism. The protoplasmic connections of the sieve plate are apparently solid, and not tubular in form. Sieve tubes of cucurbits may be grouped in 2 classes (a) central tubes of the phloem and (b) peripheral tubes of the phloem, commissural, entocyclic, and ectocyclic tubes. The central tubes have short, broad elements, exhibit slime plug formation and develop definitive callus under certain conditions. Tubes of the 2nd type vary greatly in length, do not exhibit slime plugs, and less commonly have definitive callus. Slime drops occur in all young sieve tubes. Slime plugs usually result from treatment with killing agents. Exudation from cut stems is practically limited to the phloem. Local chilling resulted in a 55.7% decrease in exudation rate. Cooling the entire plant caused a greater reduction in rate. Exudation from peduncles and young stems was very rapid. Developing fruits had a dry wt. of about 3.9%. Exudate from these fruits had a dry wt. of 7.7%. Exudates from pumpkin and cucumber had an osmotic concentration equivalent to about 7.0 atmospheres. Exudation from cut peduncles cannot be explained on the basis of mass flow through perforated sieve plates. Sieve tubes of cucurbits increase greatly in permeability with maturity. In view of this fact it seems possible that movement may take place partly through sieve tube lumina and partly through phloem walls. The relative importance of these 2 channels would depend upon the resistance presented by each to mass flow of solution.