A Translocation Hypothesis based on the Structure of Plant Cytoplasm

Two types of linear structures have been seen in plant cytoplasm,largely by phase-contrast microscopy. Microscopic fine threads,o.Iµ to Iµ in diameter, were revealed in hair cells,where they formed endoplasmic systems along streaming pathwaysin the parietal cytoplasm and in transvacuolar strands. Duringcirculation streaming, small plastids and mitochondria-likeparticles were observed moving along the fine threads. Similarfine threads, together with small plastids and mitochondria-likeparticles occurred in phloem exudate and transcellular microscopicstrands. The transcellular strands, Iµ to 7µ indiameter, were seen in sieve-tube elements, phloem parenchyma,border parenchyma, and cortical cells. The movement of small plastids across end walls in border-parenchymacells, the appearance of the same structures within strandsin phloem cells, and the undiminished occurrence of small plastidsin successive drops of phloem exudate are collectively takenas evidence for their participation in translocation. Particlemovement is thought to occur through transcellular strands inassociation with fine threads, and to be motivated by a transcellularform of protoplasmic streaming.     

The Protoplasmic-Streaming Theory of Phloem Transport

The protoplasmic-streaming theory of phloem transport has beenextended to include a transcellular streaming concept. Phloemexudate, concentration gradients, and the respiratory energyrequirement are interpreted in terms of this concept. Physiological and cytological experiments are considered, andit appears that none of the unequivocal results contradict theproposed model. Reconciliation of apparently conflicting evidenceis possible because transcellular streaming is primarily a fluidflow phenomenon and is directly dependent on respiration forits motive force energy. As in other theories, it is difficult to understand how themeasured rates of mass transfer occur through the phloem. Thepresent model may be able to meet quantitative demands becauseof increased efficiency due to the direct linear pathways providedby transcellular strands, and the increased cross-section ofsieve-tube cytoplasm. More facts are required before the masstransfer problem can be resolved.     

The Preservation of Plant Cells, particularly Sieve Tubes, by Vacuum Freeze-drying

A vacuum freeze-drying apparatus is described. Cell contentswere preserved in small, rapidly frozen pieces of plant tissuedried for four hours at – 30° C. After drying, specimenswere either directly embedded with the resin ‘Epikote’or fixed with 2 per cent osmium tetroxide in benzene and subsequentlyembedded in ester wax or Epikote resin. Mesophyll cells and border parenchyma cells were preserved inleaf pieces, and cell contents are comparable with the protoplastsof living cells. In soybean leaf, files of parenchyma cellsoccur between palisade and spongy mesophyll cells, linking fineveins. Hand sections of frozen-dried Epikote-embedded petiolephloem from Primula obconica revealed a mature sieve tube containingstrands. Preservation by freeze-drying is taken as conclusiveevidence for the existence of transcellular strands in livingsieve tubes.     

Strands in Sections of Sieve Elements Cut in a Cryostat

Observations of these strands are consistent with the idea of trans-cellular strands of cytoplasm in sieve tubes.     

Water loss from cut grass with special reference to hay-making

Water loss from cut grass was studied to determine factors limiting the drying process. An apparatus, used to measure water loss at 28±1°C from blotting paper and from leaves and stem internodes of cocksfoot, consisted of four channels in which air speed was controlled at 25–80 cm s^-1 and relative humidity at 7–68%. The maximum rate of water loss from wet blotting paper was 10500 mg water dm^-2 h^-1 but from leaves and stem internodes supplied with water it was less than 250 mg dm^-2 h^-1. The rate of loss from both plant specimens and blotting paper was linearly related to the vapour pressure differences between the specimen and the surrounding air but was not increased when air speed was changed from 40 to 80 cm s^-1. Grass specimens supplied with water had lower rates of water loss than wet blotting paper because of tissue resistances which were calculated for (a) untreated leaf and stem specimens, (b) rubbed leaves, (c) cut leaves, (d) leaves exposed to steam for 60 s. Treatments (b)-(d) greatly reduced tissue resistances. The rates of drying of leaves and stem internodes not supplied with water changed only slightly in response to faster air speeds but were significantly increased by treatments (b), (c), (c_s) (split stems), (d) and (e) (exposure to petroleum vapour for 60 s). The most effective treatments trebled the drying rates of leaves and increased the drying rates of stem internodes by 10 times. Reductions in relative humidity had little effect on drying rate following treatments (a), (b), (c) and (d), but when treatments (c_s) and (e) were given, additional significant increases in drying rates were obtained when the relative humidity was reduced. Grass specimens given the most effective treatments and dried under the most favourable conditions did not utilize the full drying capacity of the environment, for the rates of water loss from these specimens were at least three times lower than those from wet blotting paper. The results indicate that high rates of drying could be achieved at 28 ^oC or similar temperatures if practical treatments were developed to remove or greatly reduce the high resistance to water loss in cut grass.     

Strands in Sieve Tubes in Longitudinal Cryostat Sections of Cucurbita pepo Stems

Rapidly frozen vascular bundles of Cucurbita pepo were cut longitudinallyin a cryostat and observed in a Nomarski microscope. Clearlydefined transcellular strands were seen traversing the luminaof sieve elements and passing through the sieve pores. Someof these were 5-8 µm wide and gave the impression of compactand comparatively rigid structures, while others, appearingas ribbons about 3 µm in diameter, seemed to have collapsedand lost their contents during preparation. Many of the thickerstrands were seen to consist of a discrete boundary and parallelsubstructural elements approx. 1 µm in diameter. The resultsare consistent with our previous work and support Thaine's theoryof sieve tube translocation based on a transcellular strandsystem.     

Fine Structure of Sieve Tubes Prepared Mainly for Observation in the Electron Microscope by a Cryogenic Method

Using a cryogenic method of fixation, electron microscopic evidencewas obtained of structures in the sieve element lumina and sieveplate pores of Cucurbita pepo which can be interpreted as constituentsof the transcellular strands repeatedly seen in the light microscope.In longitudinal sections these appeared as more or less hollowtubes with clearly defined filamentous walls which showed someorganized substructure. Transverse and oblique sections revealedcircular or elliptical profiles of boundaries surrounding largelyempty central areas. Owing to the still inadequate preservationof specimens, the centres of the strands contained only sparsecytoplasmic material with an occasional indication of fibrilsin parallel arrangement. Improvement of fixation techniques,with rapid freezing as the preferential method, is seen as themain task in any effort to preserve the in vivo state of thesestructures.