This page has been based mainly on part of P.R. Wycherley's Chapter: The genus Hevea - botanical aspects in Sethuraj and Mathew's Natural Rubber.

Fresh Hevea latex is a polydisperse system in which negatively charged particles of various types are suspended in an ambient serum (C-serum). The two main particulate phases contained in Hevea latex are rubber particles constituting 30-45% and lutoid particles (10-20%). The third type, on a quantum basis, is the Frey-Wyssling complexes.

Rubber particles

The rubber particles usually have a size ranging from 50 Å to about 30.000 Å (3 µ). although extreme cases having 5 or 6 µ are also found. They are spherical bodies in young trees and potted plants but in mature trees the particles are large, often having a pear shape. The shape in certain cases seems to be a clonal character. Pear shape is reported to be very frequent in clones such as Tjir 1 and PR 107 (Southorn, 1961).

Particle size of greater proportion is beyond the limit of the resolution power of light microscope and reliable information in this regard could be obtained only with electron microscopic observations. Tempel (1952) recorded 1000 Å size at maximum frequency. This was confirmed later by Gomez and Moir (1979). Schoon and van der Bie (1955), observed a multi-modal distribution of rubber particles In latex of mature Hevea trees and proposed that larger particles are formed by the association of smaller particles. Gomez (1966) found a multimodal distribution in latices from young potted plants. in laticifers of very young plants small osmiophylic particles are seen freely in the cytoplasm. The structure and colloidal properties of Hevea latex has been well studied (Cockbain and Philpott, 1963; Ho et al., 1976).

A rubber particle of average size, about 1000 Å, contains hundreds of molecules of the hydrocarbon and is surrounded by a surface film of proteins and lipids. The rubber particles are also associated with triglycerides, sterols, sterol esters, tocotrienols and other lipids. Dupont et al. (1976) have confirmed the presence of phosphatidylcholine and small amounts of phosphatidyl ethanolamine in the lipids associated with rubber particles. The protein envelope of rubber particles is visible in sections of osmium stained rubber particles and is approximately 100 Å thick (Andrews and Dickenson, 1961). The envelope carries a negative charge and confers colloidal stability to the rubber particles.

According to Dickenson (1969) there are rubber particles with variously stained regions. An inner osmiophilic region surrounded by a weakly stained periphery is attributed to lack of uniformity when rubber particles are deposited on existing particles during biosynthesis. He has also suggested that the inner particulate inclusion, having 50-80 Å thickness, might be molecules of rubber of molecular weight about 100,000 but further investigations are needed to confirm this.

Lutoids

Lutoids form the next major component of Hevea latex. They are membrane-bound bodies and are mostly larger in size than the rubber particles. They are 2-5 µ in diameter bounded by a unit membrane of about 80 Å thickness. It was Wirersum (1957) who first suggested that the lutoids behave like vacuoles due to stainability with neutral red. Though controversy existed in this regard, the work of Ribaillier et al. (1971) provided evidence for the vacuolar properties of lutoids.

The content of lutoids (B-serum) has a very rapid flocculating action on aqueous suspension of rubber particles in latex, resulting in the formation of microfloccs (Southern and Edwin, 1968). This activity is apparently moderated by the ambient C-serum and Is much reduced if B-serum is boiled. Southorn and Yip (1968) demonstrated that this fast initial flocculating action of B-serum Is an electrostatic one Involving the interaction between the cationic contents of B-serum and the anionic rubber particle surface.

By phase contrast microscopy and application of suitable staining procedures the structure of lutold particles have been studied in detail. Mainly two types of fibrillar structures have been described. The first type, known as microfibrils, are characteristic of latex vessels in young tissues (Dickenson, 1965, 1969; Audley, 1965, 1966). As seen by phase contrast microscopy of tapped latex from young tissue, the microfibrils are freely suspended in the fluid content of the lutoid B-serum. The micro-fibrils are seen usually as grouped together in bundles. Each bundle has a diameter of 450-500 Å. Individual microfibrils are several micron long and 70-80 Å in diameter.

The microfibrils can be isolated from the sediments of latex from young tissues which on negative staining with phosphotungstic acid shows further details. Each microfibril is a tightly coiled continuous belix with a hollow axis. The diameter of the helix is about 125 Å and that of the hollow axis 30 Å. The microfibrils consist of an acidic protein while nucleic acid seems to be absent. Microfibrils however are not present in tissue or latex collected from the mature bark. It is believed that they disintegrate as the particles mature or else the young lutoids containing microfibrils themselves disintegrate as the tissue ages and are replaced by a population of lutoids without microfibrils. However, the microfibrils do not seem to have vital role in rubber biosynthesis.

The second type of fibriliar structures, observed in lutoids of latex from mature bark of stimulated trees, are known as 'microhelices', so named (Gomez and Yip, 1975) because of their spring like shape. These structures were first observed by Dickenson (1965, 1969). They are occasionally found in unstimulated trees and their number increases on dilution. However, microhelices are reported to be more frequent in lutoids of tapped latex than in situ latex (Southorn and Edwin, 1968; Gomez and Yip, 1975) and are occasionally observed in latex collected from young tissue also.

As reviewed by Gomez and Moir (1979) the microhelices are approximately 1 µ in length with a diameter of 200 Å, having a fibre width of about 50 Å and an open hollow helix having a 300 Å wide pitch. Dickenson (1965) suggested the formation of microhelices from microfibrils but this has been questioned by Gomez and Yip (1975).

A third type of lutoid inclusion - minute spherical particles in Brownian movement - was observed by Schoon and Phoa (1956). Later Southorn (1960, 1961) found such particles In large numbers in the bottom fraction of ultra-centrifuged latex of long rested trees and this was confirmed by Dickenson (1969). The role of such particles in latex is unknown.

Frey-Wyssling complexes

Yellow globules, in clusters In tapped latex were first noted by FreyWyssling (1929). The existence of such particles in groups, associated with a vacuolar body was observed by Southorn (1969) in phase contrast microscopy and he found that the individual particles are covered by a membrane: this was confirmed by electron microscopy. Dickenson (1969) named these particles, enclosed as a single structure, as Frey-Wyssling complexes.

The Frey-Wyssling complexes are more or less spherical in shape in a size range of 3-6 µ (diameter) and are bounded by a double membrane. Within the membrane there are two types of particles - large osmiophilic globules in variable numbers and a system of rope-like tubules of about 750 Å diameter, usually embeded in a membrane bound matrix of osmiophilic nature. The complex structure of Frey-Wyssling complexes has been elaborated by Dickenson (1969) who described a series of concentric lamellae of the double unit membrane and the system of tubules and also highly folded invaginations of the inner membrane.

The Frey-Wyssling complexes are considered to have vital role in metabolic activities. Though Dickenson (1969) opined that these structures may be possible sites of rubber biosynthesis, the double membrane and presence of carotene and polyphenoloxidase in the Frey-Wyssling complex led to a tentative suggestion that it is a type of plastid.

Types of Laticifers

Gomez (1976) made a comparative study of latex vessels collected at different stages of development or from different positions of a tree and identified mainly five types. The first type was an embryonic vessel in leaf petiole at a stage prior to the fusion of laticifer initials. This resembled a normal living parenchyma cell in cell contents, except for the presence of numerous osmiophilic rubber particles.

A typical latex vessel from the secondary phloem of green stem had osmiophilic rubber particles, ranging from 100 Å to 5000 Å in diameter. Lutoids were prominent and they contained microfibrils. This type of vessel contained mitochondria and occasionally Frey-Wyssling complexes, golgi bodies and chloroplasts.

The third type was the latex vessel collected from the secondary phloem of the tree trunk at mature age, from the inner portion of the bark, i.e., within 1 mm from the wood. This contained numerous osmiophilic rubber particles of smaller size (50 Å-2 µ) in diameter. Lutoids were present but were devoid of microfibrils. Mitochondria were also present.

The fourth type was latex vessel under tapping and had rubber particles in very large numbers. Lutoids and Frey-Wyssling complexes were common and occasionally mitochondria and endoplasmic reticulum (at the periphery) were present. Rarely, nuclei were also detected.

The last type represented senescent vessels in the outer bark. In this type, the rubber particles were comparatively larger in size and the other organelles obscured.

REFERENCES

Andrews, E.H. and Dickenson, P.B. 1961. Preliminary electron microscope observations on the ultrastructure of the latex vessel and its contents in young tissues of Hevea brasiliensis. Proc. Nat. Rubb. Res. Conf. 1960, Kuala Lumpur: The Rubber Research Institute Malaya, 756 pp.

Audley, B.G. 1965. Studies of an organalle in Hevea latex containing helical protein microfibrils. Proc. Nat. Rubb. Prod. Res. Assn Jubilee Conf., Cambridge, 1964. L. Mullins (Ed.). Maclaren & Sons, London.

Audley, B.G. 1966. The isolation and composition of helical protein microfibrils of Hevea brasiliensis latex. Blochem. J. 98: 335 pp.

Cockbain, E.G, and Philpott, M.W., 1963. Colloidal properties of latex. In: L, Bateman (Ed.). The Chemistry and Physics of Rubber-like Subtances, Maclaren & Sons Ltd,, London.

Dickenson, P.B., 1965. The ultrastructure of the latex vessel of Hevea brasiliensis. Proc. Nat. Rubb. Prod. Res. Assn Jubilee Conf., Cambridge, 1964. L. Mullins (Ed.). Maclaren & Sons, London, 52 pp.

Dickenson, P.B., 1969. Electron microscopical studies of latex vessel system of Hevea brasiliensis. J, Rubb. Res. Inst, Malaya, 21: 543.

Du Pont, J., Moreau. P,, Lance. C. and Jacob, J,L,, 1976. Phospholipid composition of the membrane of lutoids from Hevea brasiliensis latex. Phytochem. 15: 1219.

Frey-Wyssling, A.. 1929. Microscopic investigations on the occurrence of resins in Hevea latex. Arch. v.d, Rubberc,. 13: 392.

Gomez, J.B., 1966. Electron microscopic studies on the development of latex vessels in Hevea brasiliensis Muell. Arg. Ph.D. Thesis, University of Leeds. Thesis submitted for the degree of Doctor of Philosophy

Gomez, J.B., 1976. Comparative ultracylology of young and mature latex vessels in Hevea brasiliensis. Proc. Int, Rubber Conf., Kuala Lumpur, 1975. Rubber Research Institute of Malaysia, Kuala .

Gomez, J.B., and Moir. G.K.J. 1979. The ultracytology of latex vessels in Hevea brasiliensis. Monograph No. 4. Malaysian Rubber Research and Development Board. Kuala Lumpur.

Ho, C,Y., Subramaniam, A. and Yong. W.M,. 1976. The lipids associated with the particles in Hevea latex. Proc. Int, Rubber Conf., Kuala Lumpur, 1975. Rubber Research Institute of Malaysia, Kuala .

Ribaillier. D., Jacob, J.L. and d'Auzac. J., 1971. Sur certains characters vacuolaires des lutoides du latex d' Hevea brasiliensis, Muell, Arg. Physiol. Veg., 9: 423.

Schoon. Th.G.F. and Phoa, K.L., 1956. Morphology of the rubber particles in natural latices. Arch. v.d. Rubberc., 33: 185.

Schoon, Th.G.F. and van der Bie, G.J. 1955. Particle size distribution in brominatad Hevea latices. J. Polymer Sci.. 16: 63 pp.

Southorn, R.A.. 1961. Microscopy of Hevea latex. Proc. Nat. Rubb. Res. Conf., Kuala Lumpur 1960. Rubber Research Institute of Malaya, Kuala Lumpur. 766 pp.

Southorn, R.A., 1960. Complex particles in Hevea latex. Nature, 188: 165.

Southorn, R.A.. 1969. Physiology of Hevea (latex flow). J. Rubb. Res. Inst. Malaya, 21: 484.

Southorn. R.A, and Edwin, E.E.. 1968. Latex flow studies. II. Influenceof lutoids on the stability and flow of Hevea latex. J. Rubb. Res. Inst. Malaya.. 20 (4): 187.

Southorn, R.A. and Yip, E. 1968. Latex flow studies. III. Electrostatic considerations in the colloidal stability of fresh latex from Hevea brasiliensis. J. Rubb. Res. Inst. Malaya. 20 (4): 201.

van den Tempel, M. ,1952. Electron microscopy of rubber globules in Hevea latex. Trans Instn Rubb. Ind., 28: 303.

Wiresum.L.K. 1957, Enkele latexprobleman. Vakbl. Biol.. 3: 17.