Sieve-tube system

Fromm and Bauer [36] found that action potentials in maize sieve tubes change phloem translocation. Using macro- and microautoradiography in mature leaves of maize, Fromm and Bauer [36] studied the inhibition of phloem translocation caused by electric and cold shock. They stimulated the leaf tip with ice water and found that the velocity of signal transmission was 3-5cm/s. Upon stimulation, the microelectrode recorded a basipetally propagating action potential with a depolarizing amplitude of 80 mV in the sieve tubes (Fig. 2). During electrical stimulation, the action potential was measured in the sieve-tube system with a speed of 5cm/s. 

The transport system of the sieve tubes is also a multiplet of generalized transport units (pTU) with the sieve plate pores as organismal capillaries. Though the sieve tubes are thin as such, the dependence of the hydraulic conductivity, on the fourth power of the radius, makes the distinction between them and the pores essential. The effects of occlusion on gw are difficult to calculate but the general features of the sieve tube system permit one to surmise that it has been made to cope with adverse hydrostatic gradients of variable but often large magnitudes. 

As in the case of the xylem conduits, the sieve tubes communicate with the cells and tissues around them. Lateral transport systems, including the pathways for the exchange of substances between them and the xylem conduits, may be visualized as lateral branches of the long distance pathways (xTU and pTU). These offshoots (ITUs) link all regions of the cross section of the root and the shoot with the xTU and pTU. 

The flux of water and solutes, 5, this time to a sink cell (/) will be again given by Eq. (33), the sources on which the summation has to be carried out in order to get the total influx being all the mesophyll cells in the shoot. For the flow of S in the sieve tubes the values of the coefficients Cpr and Cel would be for the system pTU. Because of the dependence of these coefficients on the inverse of the distance between the source and sink, only the sinks close to a leaf can draw S towards them. However, the amount of S pulled by such a region of sinks, pTU is more... 

Endogenous growth regulators may be translocated in the plant s vascular systems. This would seem to hold especially true for gibberellins and cytokinins, since activity has been detected in the xylem sap of many different herbaceous as well as woody species. Obviously these hormones are also exported from photosynthesizing leaves, as indicated by their presence in sieve tube sap. Although with less frequency, also auxin and ABA-like activity have been detected in both xylem and phloem sap (Table 3.1). 

Plants intact except for the transverse wound regenerated numbers of vascular cells equivalent to those formed by about 0.05% lAA added to the isolated internode (asterisks in Fig. 4.9). Hence, 0.05% lAA in lanolin exactly replaced the root system and the rest of the shoot system in their effects on the regeneration of both sieve tubes and tracheary cells. At any one lAA level, more sieve cells than tracheary cells regenerated. 

The mechanism of transport in the sieve tubes has still not been elucidated. Nonetheless most of the presently known findings argue in favor of the correctness of the mass flow hypothesis put forward by Munch in 1926. According to it, convection or mass flow is responsible for the transport in the sieve tubes just as for transport in the xylem or in the blood vessel system of animals. The driving force of this mass flow in the sieve tubes is a concentration gradient of osmotically active substances decreasing in the direction of transport. 

The second analytical method uses a combustion system (O Neil et al. 1994) in place of reaction with BrF,. This method was used for the crocodiles because they were represented by very thin caps of enamel. The enamel was powdered and sieved (20 mg), pretreated in NaOCl to oxidize organic material and then precipitated as silver phosphate. Approximately 10-20 mg of silver phosphate were mixed with powdered graphite in quartz tubes, evacuated and sealed. Combustion at 1,200°C was followed by rapid cooling in water which prevents isotopic fractionation between the CO2 produced and the residual solid in the tube. Analyses of separate aliquots from the same sample typically showed precisions of 0.1%o to 0.4%o with 2 to 4 repetitive analyses even though yields are on the order of 25%. 

RF-Plasma Torch Reactor. A diagram of a radiofrequency (RF) plasma torch reactor (25) is shown in Figure 8.1.3. The apparatus consists of an RF-plasma reactor with frequency of 4.0 MHz, a powder feeder, a gas supply system, and an exhaust system. The torch is composed of a work coil and three concentric quartz tubes for three independent gas flows outer, inner, and center carrier gas flows. For TiN UFPs synthesis, N2 is added to the outer flow of Ar, and titanium powder sieved in advance to a size less than 25 p.m is fed into the plasma by the carrier gas of Ar... 

Two complex tissues, the xylem and phloem, provide the conducting network or "circulatory system" of plants. In the xylem or woody tissue, most of the cells are dead and the thick-walled tubes (tracheids) serve to transport water and dissolved minerals from the roots to the stems and leaves. The phloem cells provide the principal means of downward conduction of foods from the leaves. Phloem cells are joined end to end by sieve plates, so-called because they are perforated by numerous minute pores through which cytoplasm of adjoining sieve cells appears to be connected by strands 5-9 pm in diameter.154 Mature sieve cells have no nuclei, but each sieve cell is paired with a nucleated "companion" cell. 

A simple sampler usually consists of a glass or stainless steel tube filled with a certain amount of the sorbent. Typically, 50-500 mg of the sorbent are used. To retain the sorbent in the tubes either glass/quartz wool or small sieves can be used. Both techniques introduce a disadvantage into the sampling system quartz wool is normally too brittle to be fitted successfully into the tube, and small parts of the... 

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