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1. Electron microscopic studies of the sieve tube sap obtained from the secondary phloem of Robinia pseudoacacia by the method of Hartig (1860) showed the presence of well developed mitochondria in addition to membrane fragments. 2. In this sieve tube sap the following enzymes could be detected qualitatively: UTP-glucose-1-phosphate-uridyl transferase, UDPG-fructose glucosyl transferase, glucose-6-phosphate dehydrogenase, hexokinase (for glucose and fructose), phosphohexose isomerase, phosphofructokinase, and UDPG-pyrophosphatase. 3. The following enzymes were determined quantitatively: phosphorylase, amylase, aldolase, triosephosphate isomerase, NAD(+)-dependent glyceraldehyde-3-phosphate dehydrogenase, phosphoglyceromutase, enolase, pyruvate kinase, pyruvate decarboxylase, alcohol dehydrogenase, isocitrate dehydrogenase, fumarase, malate dehydrogenase, glutamate-pyruvate transaminase, glutamate dehydrogenase, glutamate-oxalacetate transaminase, and anorganic pyrophosphatase. 4. The following enzymes could not be detected: UDGP dehydrogenase, UDPG-fructose-6-phosphate-glucosyltransferase, invertase, phosphoglucomutase, lactate dehydrogenase, and citrate synthase. 5. The enzyme pattern in the sieve tube saps of Tilia platyphyllos, Carpinus betulus, Fraxinus americana, Quercus borealis maxima, and Salix viminalis is qualitatively similar to that of Robinia, but shows quantitative differences (as far as analyzed). 6. The meaning of the results for the metabolism and function of the sieve tubes in situ is discussed.

Two phosphorylases could be detected on gel-electropherograms of leaf-extracts of Spinacia oleracea and of immature cotyledons of Vicia faba. These two phosphorylases could be separated by means of ammonium sulfate fractionation. Both the slower migrating phosphorylases from spinach and from beans, but not the fast one from beans, could be adsorbed on amyloplasts. This process takes place only when the amyloplasts are suspended in a salt medium. The slow phosphorylases can also be adsorbed on chloroplasts. The specific activity of the amyloplast-adsorbable phosphorylase in spinach leaves is about ten times higher in the cytoplasmatic fraction than in chloroplasts, a fact which suggests that this phosphorylase is localised in the cytoplasma. The addition of ADP or ATP to the reaction mixture had no influence on the synthesizing activity of the slow phosphorylases when they were tested with soluble amylopectin as a primer or while they were adsorbed on amyloplasts. The presence of ADPG and UDPG was inhibitory.The results reported above suggest that phosphorylase could catalyse the synthesis of starch in the plastids when photophosphorylation or oxidative phosphorylation occurs. This starch synthesis could be controlled by the concentration of ADPG. When, on the other hand, the ATP/Pi ratio is low, phosphorylase would be involved in starch breakdown. This reverse reaction is also regulated by the concentration of glucosylnucleotides.

Starch synthesis in potato tubers grown at varied K nutrition (0.1 (K1), 0.25 (K2) and 1.0 mmol K L-I nutrient solution (K3) was investigated with particular regard to the activity of selected enzymes (sucrose synthase, UDP-D-glucose pyrophosphatase, starch phosphorylase, amylases) in dependence on tuber K content. Allocation of K to the tubers was nearly the same in all treatments. The activity of enzymes related to tuber K content did not differ significantly. Starch and K content of tubers increased with progressing age, whereas a decrease was observed in growth rate, starch synthesis per day and K uptake per day. Positive correlations between the rates of K uptake, starch production and growth indicate that the dynamic phase of K supply to the tubers is of greater importance for starch synthesizing processes than the influence of total K content.