Laticiferous plants of economic importance

Laticiferous Plants of Economic Importance I. Sources of Balata, Chicle, Guttapercha and Allied Guttas

L L E W E L Y N W I L L I A M S 1

Gutta-gums figure prominently in internationM trade. For more thorn I00 years the United States has been, and continues to be, the principal cousumer of these raw materials. Trade reports issued by the U. S. Department o[ Commerce indi- cate that during 1960 approximately 11,717,000 pounds of these forest products, with an aggregate ralue of $6,:$36,792, were imported, mostly from Malaysia, Central America, and northern South ,4merlca. These commodities are still furnished mainly by "wild" trees.

Plantation development has been limited to small-scale propagation of guttapercha (Palaqulum gutta Burck) in Indonesia and Malaya and recently of chide (Cnidoseolus spp.) in western Mexico. In comparison with Hevea brasiliensis (Willd. ex Adr. Juss.) Muell. .4rg., Castilla elastica Cerv., and other species of rubber, relatively little has been published on the sources of gutta-gums.

I n t r o d u c t i o n

In gener-fl usage, the term "gum" is applied ambiguously to a wide range of natural substances obtained from latices or as exuda- tions from bark, stems, branches, fruits or seeds. These secretions may be grouped into various classes, according to their plant ori- gin, physical and chemical properties, and industrial uses.

This discussion is restricted to materials obtained from the inspissated latex of plants~ in which the principal components are hydrocarbons and resins, in addition to a series of secondary ingredients in suspension or dispersion. When latex is coagulated, the resulting product nmy be differentiated into two m-fin types :

(a) Rubber, in which the coagulum contains a high percentage of caoutchouc-hydrocarbon, a eis-polyisoprene, the component which im- parts elasticity, usually associated with and characteristic of rubber. The word "caoutchouc" is derived probably from "caucho" or "eahuchu," the local names by which a rubber tree (CastilIa ulei Warb.) and its product have been known since pre-Columbian times in the upper Amazon basin.

(b) Gutta-gum, isomeric with the caoutchouc-hydrocarbon, in that one of the principal components is a gutta-hydrocarbon, a trans-polyisoprene. This type of product, with which the present discussion is pr imar i ly

1U. S. Department of Agriculture, Agricul- tural I~eseareh Service, Crops tleseareh Divi- sion, Bcltsville, _~[aryland.

17

higher proportion of resins to hydrocarbon concerned, contains usually a considerably than does rubber. On drying, the coagulum, obtained by processing the latex, hardens usually into a solid mass~ or, in inferior types of gutta, it often becomes friable. The best grade, regarded as a prototype of this material, is guttapercha~ obtained from a tall tree (Palaq'Mnm gutta Burck)~ indigenous to southeast Asia.

Natural latices and their coagulated products have attracted the attention of botanists, foresters and part icularly of chemists for many years. The latex of the Pardi rubber tree (Hevea brasiliensis), for example, and to a less extent that of the hule tree (Castilla elastica) of Mexico and Central America, and guayule (Parthenium argentatum A. Gray) of northern Mexico have been the subject of considerable field and laboratory researches, and the list of publications on these plants is exceedingly extensive. On the other hand, ex- cepting studies by Kar l ing (4) on the sapodilla tree (Achras zapota L.), the Fores t De- partment of Malaya on jelutong (Dyera spp.) and the Department of Agriculture of Netherlands Indies on guttapercha (Pala- qu i ,m spp.), information on the latices and products of many gutta-yielding species is fragmentary.

The purpose of this contribution is to as- semble our knowledge of the sources and distribution of plants furnishing gutta-gums, part icularly those of economic importance, together with data on yield, methods of tapping and coagulating the latex, and the proc-

18 ECONOMIC BOTANY

essing of the raw material. The fact that these products continue to figure prominently in in- ternational commerce indicates the need for such a compilation. But, before describing at length the sources of gutta, it might be well to discuss briefly the occurrence of latex in plants, its composition, methods of extraction and uses.

Latex The designation latex is applied to the fluid,

usually whitish, in specialized cells or vessels in many species of plants of diverse families. More precisely, it is a substance consisting of a li(tuid nmtrix or serum with minute organic particles in dispersion or suspension, which are of variable size, and spherical to rod-or pear-shaped. The structures containing latex are of variable size, and spherical to rod- or

series of fused cells, or a cell-system, often tube-like and complex, that permeate various tissues of the plant (see Figs. 1, 2). They may be present in the cortex, pith, occasionally in the wood, also in the leaves or roots, and they may be found in the seedling as well as in the fruit.

The adjective laticiferous, from the Latin btc (milk), is often applied to latex-yielding plants. Since latex is highly variable in composition and is not always necessarily milk- or cream-like in appearance, Esau (1) suggests the more applicable term laticiferous, derived from latex, in Latin meaning juice. To sim- plify this further, it seems that the term lati- ferous would also be acceptable. Iu.~tead of lactiferous duct or tube, Jackson (3) suggests laticifeJ: because of its simplicity. This term

A

! !

H

G

F

Fig. 1. (Left) Laticiferous system of Achra~ zapota L. A.. Portion of incipient latex vessel in the pith with partly absorbed transverse septa (1-7). B-G. Stages in the absorption of transverse walls between latex cells. H. Portion of a fairly mature latex tube in pith, with cross wall almost completely disappeared and end of original cells considerably enlarged. (Adapted from Karling, Amer. Jour. Bot. 16. 1929. Dra~wa by Regina O. Hughes.)

:Fig 2. (Right) Diagram of phloem and adjacent tissues of Hevea brasiliensis, showing arrange- ment of articulated taticifers in the secondary phloem. 1) cork; 2) cortex; 3) sclereids; 4; ray; 5) laticifers; 6) xylem. (Adapted from X=ischer, 1923. Drawn by Regina O. Hughes.)

LATICIFEROUS PLANTS OF ECONOMIC IMPORTANCE 19

appears to be more descriptive when applied to a single cell, a structure resulting from fusion of cells, or an anastamosing system. A single-cell laticifer may be qualified as a simple laticifer, while the structure evolved from the union of cells or a branching system may be classed as a compound laticifer. The concept and classification of laticifcrs are well described by Esau (1).

P l a n t S o u r c e s

Although laticifers are most prevalent in the dicotyledons, they are restricted to a limited number of unrelated families. Of these the families best represented by laticiferous plants arc Apocynaceae, Aselepiadaccae, Carieaceae, Compositae, Euphorbiaceae, Moraceae, and Sapotaccae.

Of the gymnosperms, only a few arc known t~ yield latex or to form rubber or gutta. Polhamus 1 reports that he has been able to extract rubber-containing latex from the peel of .lIus¢~ .~'apieut~rm L. Rubber has been extracted also, according to him, from a fungus attacking the roots of a certain tree in the Fa r East.

The number of latex-yielding species has been roughly estimated at 35,000 to 38,000, according to Sehultes, 1 of which more titan 7,000 species contain caoutchouc. Moyle (6) lists 1,$00 rubber-yielding plants.

H'ersonal communication, 196].

The great majority of these plants produce only a small volume of latex or a minute quantity of gutta or rubber (caoutchouc), in- sufficient to be of economic value. In others, the yield of latex may be high, but with only a low concentration of solids. In some plants, such as species of Hevea or Palaquium, the solids (hydrocarbon and resins) nmy range up to 50 or 60% of the total weight of the latex.

Although the capacity to form latex occurs among plants in no definite form or pattern, certain facts are apparent. While laticiferous plants arc found in most parts of the world, most arc of pantropical distribution (Fig. 3).

"They range from small herbs, such as species of Euphorbia, to stout woody vines (Campsi- andr(l, Clitandra, and Landolphia) of central and west Africa, the gregarious rubber trees (Herea) of the Amazon basin, or the giant jelutong tree (Dyera costulata Hook. f.) of southeast Asia.

All genera within any one of the families imlicated do not necessarily furnish gutta or caoutchouc, and even within the same genus may differ greatly in their capacity to form latex. For instance, species of a certain genus in one climate nmy accuumlate a considerable amount ol latex, whereas other species of the same genus in a different climate may form little or no latex.

Species of economic value are most abun- dant in tropical regions; in temperate zones

A /-

Fig. 3. Although laticiferous plants grow almost throughout the world, genera of economic importance (Castilla, Couma, Dyera, Funtumi(t, Hevea, Manilkara, nnd Palaquium) are concentrated mainly in tropical regions.

20 ECONOSIIC BOTANY

only a relatively few plants furnish latex in sufficient volume, or a product of suitable quality to be potential sources of raw materials. These are guayule (Parthenium arge~t~rtum) ; kok-saghyz and related species of Tara.racum; and perhaps Asclepius sub~- lata Decne., and A. erosa Torr. The possibili- ty, however remote, that new sources will be discovered should not be discounted. The dis- covery of kok-saghyz (Taraxaeam kok-saghyz Rodin) in the Tien Shan mountains of Rus- sian Central Asia occurred only about 30 years ago.

Composition. Like cell sap, latex contains an assortment of substances in solution and in colloidal suspension. Minute granules or particles in suspension are represented by such ingredients as hydrocarbons, resins, essential oils, balsams and camphors. In addition, car- bohydrates, organic acids, mineral salts, al- kaloids, sterols, fats, tannins and nmcilages may be present. Of these substances, the hydrocarbons, sometimes referred to as gutta or caoutchouc (according to the base material) and particularly the resins are the principal components of latex in many plants. Resins occur in vaITing amount, depending upon the species. The fact that a plant yields latex is not always indicative of the presence of a hydrocarbon (gutta or caoutchouc). Many latices, in fact, lack a hydrocarbon completely but contain nminly such suspended matel~als as proteins, waxes and esters. A notable instance of this is the latex of Carica I'ap~ya L., in which the most valued ingredient is papain, containing protein-digesting en- zymes.

Latex varies considerably in volmne, appearance and chemical composition from species to species, even from tree to tree of the same species~ and at different times of day or period of year. The yield and quality of latex nmy also be influenced by the microclimate, soil conditions or altitude. We are, therefore, not .justified in considering the latex of one particular species as being typical of all latices.

Possible Function of Latex A number of hypotheses have been profer-

red on the function of latex as a whole or of any one of its components (2, 5, 7). Despite considerable research its purpose remains obscure. Some theorize that latex is a means

of sealing wounds, as protection of the plant against attacks by insects or other predators. This view is discounted by certain investigators. According to the author's observations on Couma macrocarpa Barb. Rodr., in the Peruvian Amazon, however, a certain protection appears to be provided by the exuda- tion to counteract, or as a reaction of, in jury caused by beetle borers.

Some investigators incline to the belief that laticifers serve for the conduction of food materials or that latex may be a storage of food reserve, but results of experiments conducted do not fully support these views. Once de- posited in the plant, latex does not appear to be mobilized even under severe conditions of starvation, such as prolonged etiolation or continued defoliation.

Another interpretation of the possible function of laticifers is that they form an ex- cretory system. Some researchers maintain that latex represents a non-functiomd by- product of cellular metabolism. I t is possible that latex may serve a certain purpose or many purposes in one plant, but it may serve no specific function in another plant.

Extraction of Latex Different methods are utilized to extract

the la tex- - l ) by mechanical means, in which process the small guayule shrub (Partbenium arc2entat~m A. Gray) of northern Mexico is macerated, or the leaves of the Malayan Palaquium are crushed; 2) a solvent to extract gutta from the leaves of Palaqtdum gutta> practised in Malaya; 3) by felling the tree and girdling the trunk, as in certain species of Manilkara; or 4) by tapping the standing tree, the accepted system of extract- ing the latex of Hevea brasiliensis.

Representatives of the genus Manilkara usually have a very thick, scaly bark and can be retapped only at long intervals of several years. Consequently the general practice in the Amazon basin to tap massaranduba (M. huberii Standley), and occasionally balata (M. bidentata (A. DC.) A. Chev.), is to fell the tree and to girdle the entire trunk, in order to extract as much latex as possible at one trine.

In tapping standing trees, such as Hevea or Cnidoscolus, the usual method is to open channels of various patterns in the cortex, to sever the laticifers and thus release the latex

LATICIFEROUS PLANTS OF ECONOMIC IMPORTANCE 21

Fig. 4. (Left) Hevea braailiensis tapped repeatedly on the panel system. Partly healed-over panel (left), tapped the previous year. River Itecoahy, Javar:~ Basin, Brazilian Amazon, 1953.

Fig. 5. (Right) Experimental tapping of Leche-caspi (Couma macrocarpa) on upper Curaray River, Napo Basin, Peruvian Amazon, 1954.

(Figs. 4, 5, 6). The method and frequency of tapping are determined by the behavior of the latex and particularly by the response of the plant to tapping, as indicated by the rate of bark renewal and capacity to replenish its store of latex. For instance, the Par~ rubber tree (Hevea brasiliensis) has a remarkable reaction to tapping and quickly restores its latex. Normally, this tree can be tapped on a single panel, confined to one face of the trunk. Tapping is repeated on alternate days, or even daily, for a period of 9 to 12 months each year. Karling and other authors refer to this as the ibidem panel system (Fig. 7). Most trees of the Sapotaceae, on the other hand, have a very poor response to tapping, as indicated by the slow recovery of the bark, and prolonged rest periods are necessary to re- store latex to an economic level that would justify retapping. I t is for this reason that trees of the genera Aehras and P~daq~dum

can be retapped only at long intervals, up- wards of 3 years.

In the process of tapping, the laticifers are severed to release the latex which flows under pressure. In the whole or uninjured plant, the laticifers are under turgor, and at the same time in osmotic equilibrium with the sur- rounding cells. In other words, the turgor pressure in the laticifer balances the osmotic pressure. When the laticifer is opened a turgor gradient is established and the latex flows towards the incision, where the turgor has been reduced to zero. When the flow finally ceases, cell renewal occurs, latex is reforTned and turgor is gradually restored. In some plants restoration takes place almost immedi- ately; in others several days, weeks or months may elapse; and in other plants there is no complete recovery.

In Hevea brasiliensis, as indicated, the cell is capable of reforming latex and in restoring

22 ECONOMIC BOTANY

Fig. 6. Six-year old red ehilte trees (Cnidoscolus elasticus) grown from seed at Atkins Garden and Research Laboratory, Cuba, tapped on a modified herring-bone system. 1953.

its turgidity with remarkable rapidity so that this tree is capable of withstanding repeated tapping at daily or alternate daily intervals. Species of Dyera and Cnidoscoltts may be retapped at intervals of 1 or 2 weeks, whereas Co,tma species can be tapped only once, or twice at the most, each year. Most species of the Sapodilla family, particularly those of the genera Achras, Manitkara and Palaquium~ can be tapped only once each 3 to 5 years.

A feature frequently observed in the initial tapping of Hevea~ Co*lrna and certain other genera is the aqueous, turbid appearance of the liquid flowing during the first few seconds. This fluid is a mixture of latex exuding from the laticifers and moisture from the surround- ing tissue, which likewise is under turgot. Once a free flow has been established, the latex becomes chalk-like, with a milky, or in some instances a viscid, consistency.

Coagulation of Latex To separate the solids from the serum or

liquid matrix, and to prepare a product for industrial use, the first step is to coagulate the latex, usually in the fresh state. Coagulation may be brought about in various ways: depending upon the behavior of the latex. In chilte (Cnidoscobts elastic~s Lundell and C. tepiquensis (Cost. & Gall.) MeVaugh), rapid coagulation is obtained simply bv allowing the latex to coagulate naturally on standing.

The latex of most species of Achras~ Co~tnta, Funtumia, Manilkara and Palaq~d~tm, which is often viscous, coagulates best by cook- ing and stirring. Some latices, sueh as those of Dgera and Hevea~ coagulate readily with mineral acids, such as acetic or phosphoric. Others may be coagulated by adding a small amount or' mineral salt, fruit-acid, sap of certain plants, cold or hot water; and even sea water is known to have been used in the Sulu Archipelago. In Haiti, according to Polhamus, it was found that CrBptostegia latex coagulates readily by adding a small volume of Hevea latex.

Several theories have been advanced rela- tive to the e'mse of coagulation (2). One hypothesis hohls that the process is induced by an enzyme present in the latex, while others maintain that a change occurs in the protein present in tim latex. But whatever the cause, the purpose of coagulation is to alter the colloidal stability of the dispersion. In some latices, coagulation nmy be accom- plished by lowering the pH of the solution; in others, it is attained by increasing the pH.

Properties of Gutta-Gums As already pointed out, the latex from

which gutta-gums are derived is a mixture of components, but most often the principal in- gxedients are a hydrocarbon (gutta) and resin. The proportion of these two components is not constant, even within the same species. Therefore, the properties of raw materials extracted from these latices are subject to considerable variation, and their re- jection or acceptance and ultimate use in in- dustry are determined primarily by the ratio of gutta to resin, although the type of resin is also important.

The fresh eoagulum of most gutta-gums is usually whitish to cream-colored, soft and pli- able. During drying and exposure of the eoagulum to all 5 the color generally darkens as a result of oxidation, and a solid, hard mass

I,ATICIFEROUS PLANTS OF ECONOMIC IMPORTANCE 23

BARK \ l "~--"~ ";2- \ ' ~ x Br.~nch

' Cork I~ Fork

5tone cell i ~--- 4

bear- Renewin~ bark Tappin~ cut in 8 cortex " ~ T~ppin~ p~nel

_Cambiu .__.-.-------- L~tex channet 5pout

~ 7 L~tex cup Cup han~er

~"S, oil level ,,,,, - . / . - - _ . , , . _ . -

• _ _ _ . : _ . _ _ _ _ . :

- _ . - - Y e ' 2 ~

Fig. 7. Bark of lIevea bra.~.ilie~lsis and the panel syste mfor repeated tapI)ing (ibidem method) within a limited area.

is formed, as in gutta-percha (Palaq~dum gutta) or balata (Manilkara bidentata). In some of the lower-grade materials, such as Cuidoscolus, Couma and Dyera, however, it becomes brittle and friable when thoroughly dried. To check oxidation and deterioration, the fresh gum may be stored in water, as in the case of Dyera in MaLaya or Manilkara in the Amazon basin, until the material can be shipped.

Unlike Hevea rubber, gutta-gums usually contain a considerably higher proportion of resins than g'utta, and are non-elastic. They are sometimes referred to as insoluble gums, since they are not materially affected by im- mersion in water. However, the resins may be dissolved in acetone, alcohol and similar chemical agents.

Up to the present, only a very limited nmn- bet of gutta-yielding trees, particularly those of the Apocynaceae and Sapotaceae, occur in stands sufficiently extensive to .justify their

utilization and produce gutta-gum in sufficient quantity and of acceptable quality for industrial use. Of these, the first to become known was gutta-percha (Palaquium gutta) of southeast Asia, long the principal source of gutta of commerce. Later bnlata (Manilkara bidentata), of the same family (Sapotaceae), originating in the Guianas and Amazon basin, came into prominence. As their properties became better known, other species attained commercial importanc% such as sapodilla ( Achras zapota), of southeastern Mexico and Central America; and jelutong or pontianak (Dyera costulata and D. laxiflora Hook. f.), tall, corpulent trees of the Apoeynaceae oc- curring in Borneo, Indonesia and ~{alaya. Additional sources of inferior guttas of recent development include leche-caspi or sorva (Co-~tma ~acrocarpa), of the Amazon basin, and red and white chilte (Cnidoscolus elasti- cu.~ and C. tepiquensis), of west-central Mexi- co. Two temperate zone plants, a species of

24 ECONOMIC BOTANY

Ett ot~/llt~tts and Eucommia ,dmeoides Oliver, have been reported to be used for the produc- tion of gut ta in the Soviet Union.

Uses of GuttarGums On the basis of their sinfilar appearance,

behavior and chemical composition, gut ta- gums and articles manufac tured f rom these raw products, par t icu lar ly gut tapereha and balata, were former ly classed in the trade with rubber. Originally, gut tapereha was regarded as an ideal substitute for rubber and was used in combination with rubber for the manufac- ture of flexible, elastic, wa te rproof articles. Bu t as its proper t ies became better known, par t icular ly the fac t that it has a high resin content combined with a lower percentage of gut ta , its appl icat ion in indust ry became re- str icted to more specialized uses.

Gut tpercha and to a less extent balata have been used almost exclusively fo r many dec- ades to cover submarine cables, since they possess high insulat ing power, dielectric s trength and low inductive capacity. They arc also utilized for the nmnufacture of transmis- sion belts and in dentures.

Fo rmer ly je lu tong or pont ianak (Dyera spp.) was used in the manufac ture of in-

fe r ior rubber articles in which elasticity was not a pr ime consideration. Since this product contains only about 20 percent gut ta: it is now utilized al,nost exclusively as a base material in the manufac ture of nmsticatories. Some of the infer ior gum materials serve as cxtenders or to blend with super ior grades of guttas.

L i t e r a t u r e C i t e d

1. Esau, Katherine. Plant Anatomy. New York, John Wiley. 1953.

2. Hauser, Ernest A. Latex. New York, Chem- ical Catalog. 1930.

3. Jackson, B. A. A glossary of botanic terms. 4th. ed. London, Gerald Duckworth. 1928.

4. Karling, 3". S. The laticiferous system of Achras zapota L. Amer. Jour. Bot. 16: 803-824. 1929.

5. Moyer, L. S. Recent advances ill the physi- ology of latex. ]3ot. I~ev. 3: 522-544. 1937.

6. Moylc, A. E. Bibliography and collected ;tbstracts on rubber producing plants. Texas Agr. Expt . Sta. Cir. 99. 1942.

7. Parkin, 3-. Observations on latex and its functions. Ann. Bot. 14: 193-214. 1900.

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Laticiferous plants of economic importance