The Physiology of the Sieve Tube

Abstract

The willow aphid Tuberolachnus salignus feeds by inserting its stylets into a single sieve element. If the insect is severed from its proboscis whilst feeding, sap from the sieve tube exudes from the cut end of the stylet canal and may continue for many hours. By attaching a capillary tube the rate of exudation can be measured and the sap analysed. Normally the rate of exudation was about 1 mm.³ per h. This means a movement of water and sucrose of 100 cm./h. or 100 sieve elements per min. The sap was found to contain between 5 and 15 per cent, sucrose, up to 0.4 per cent, raffinose, no reducing sugars, and about 0.5 per cent, amino-acids. Girdles some distance above and below exuding stylets failed to stop exudation (Tables I and II). This indicates a rapid sealing of the cut ends of the sieve tube and a switch-over in source of supply from the leaves to storage cells in the stem. The use of isolated stem segments and even irrigated strips of bark followed. Using these, the following points have been established, (a) There is no polarity of movement (Table IV); (b) a minimal length of about 16 cm. of stem or 800–1,000 sieve elements are needed to give the full exudation rate (Fig. 5); (c) when the D.P.D. on the inside of the bark is raised (Figs. 6, 7, and 8), the rate of exudation declines, but the concentration of the exudate rises and exudation continues, indicating a maintenance of turgor, even in face of a D.P.D. of 20 atm. or more. Secretion of sugar into the sieve tube continues albeit slowly even against a sieve tube concentration of 50 per cent. These findings are discussed in relation to current theories of translocation and are considered to favour the mass flow hypothesis.