Typology of foliar tracheoids in angiosperms

Proc,. Indian Aead. Sci., Vol. 88 B, Part II, Number 5, September 1979, pp. 331-345, @ printed in India.

Typology of foliar traeheoids in angiosperms

T ANANDA RAO and SILPI DAS Botanical Survey of India, Howrah 711 109

MS ro0oived 18 May 1978; revised 23 July 1979

Abstract. A resum0 on tim morphological features of trachooids has boon presented under various typological heads with examples drawn from the published literature to enhance their utility in detailed descriptions of tracheoids. Depending on the constancy of cell form the traehooids are classified under two groupings : homomorphic and hetoromorphic. Homomorphie group includes brachytracheoids, selero- tracheoids, macrotrachooids, tubuliform spirotrachooids, vermiform trachooids and loratotrachooids. Hetoromorphie grouping includes heterotracheoids. Each type represents a descriptive unit recognised on the cell form and is supported by a quali- fying description for their delimitation.

Keywords. Foliax tracheoidsi enumeration; typology.

1. Introduction

The early researchers dealing with this aspect of systematic anatomy reported enlarged terminal tracheids either at the terminus of vein-endings or away from them in the leaves of several taxa attributing to them the function of storage of water (Vesque 1882; Heinricher 1885; Gilburt 1881; Mangin 1882; Kny and Zimmermalm, 1885). Solereder (1908) enumerated 28 families of dicotyledons in which around 76 taxa have enlarged tracheids. Further, he considered the enumeration as incomplete in view of the term enlarged terminal tracheids being not always interpreted in the same sense by different authors. In most of the cases the terminology used is inexact and insufficiently descriptive. Since then there has been resurgence of interest in such cells in comparative morphology (Haberlandt 1914; Pirwitz 1931; Pass 1940; Lloyd 1942; Foster 1946, 1947, 1956; Rao 1957; Rao and Mody 1961; Tucker 1964; Bierhorst and Zzmura 1965; Bokhari and Burtt 1970; Dickison 1969, 1970, 1973a, b; Henrickson 1972; Govindarajalu 1972; Govindarajalu and Parame',,waran 1967; Napp-Zinn 1973 ; Sehgal and Paliwal 1974; Larsten and Carvey 1974; Bokhari and Hedge 1975; Rue and Das 1976; Pant and Bhatnagar 1977).

An enumeration of tracheoid idioblasts bearing taxa in angiosperms mostly retrieved from the works of Soloreder (1908, here abbreviated, Sol.) and Metcalfe and Chalk (1957, here abbreviated, MC) and from the many recent publications of other authors is presented in this paper, which is intended to help in recognising

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332 T Ananda Rao and Silpi Das

the genera in which tracheoids have been reported in the past. It is clear from the records that many citations from Sol. and MC and other authors need to be examined afresh for a clearer picture of the tracheoidal typology and their structural features.

2. Enumeration of foliar tracheoids bearing taxa in angiosperms (Dahlgren's system t975)

2.1. Dicotyledons

Magnolianae : Magnoliales: Magnoliaceae--Kmeria, Liriodendron, Magnolia, Manglietia, Miehelia, Taluma (Tucker !964).

Ranunculanae: Rutales: Rutaceae--Boronia (Sol. p. 854; Rao and Bhatta- charya 1978), Phebalium (Sol. p. 854). Simaroubaceae-.Hannoa (Foster 1956).

Polygalales : Vochysiaceae--Vochysia (Sol. p. 1093; Rae and Mody 1961). XanthophyUaceae--Xanthophyllum (Dickison 1973b). Polygalaceae--Badiera, Monnina, Polygala (Sol. p. 96). Krameriaceae--Krameria (unpublished).

Geraniales : Averrhoaceae--Averrhoa (Sol. p. 851). Oxalidaceae--Biophytum, Eichleria (Sol. p. 851).

Asteranae : Asterales : Asteraceae-Aster, Mulgedium, Tussilago (Sol. p. 460) Centaurea (HeinriGher 1885), Helianthus (Alexandrov 1925).

Dillenianae : Dilleniales : Dilleniaceae--Hibbertia (Dickison 1970).

Euphorbiales : Euphorbiaceae--Amanoa (Sol. p. 749), Euphorbia (Haberlandt 1914; Sehgal and Paliwal 1974), Pogonophora (Sol. p. 749; Pirwitz 1931; Foster 1956).

Violanae: Tamaricales : Tamaricaceae--Reaumuria, Tamarix (Sol. p. 114; MC, p. 157; Vesque 1885).

Capparales : Salvadoraceae--Azima (Govindarajalu and Parameswaran 1967), Dobera, Platynitium (Sol. p. 527), Salvadora (Sabnis 1921). Capparaceae-- Capparis (Sol. p. 581; Gramse 1907; Bokhari and Hedge 1975; Rao and Das 1978).

Celastanae : Celastrales: Avicenniaceae--Avicennia (unpublished). Celastraceae --Maytenus, Polycardia, Schaefferia (Sol. p. 877). Goupiaceae--Goupia (Sol. p. 877; Rao and Bhattacharya 1975).

Santalales : Olacaceae--Agnandra, Schoepfia, Phalbocalymus, Ximenia. Leran- thaeeae--Loranthus (Rao and Kelkar 1951), Nuytsia (Sol. p. 727). Viscaceae-- Viscum (Sol. p. 727).

Hamamelidanae : Casuarinales : Casuarinaeeae-Casuarina (Sol. p. 887).

Rosanae : Rosales: Rosaceae--Moquilea (Sol. p. 303). Connaraceae--Pseudocon- narus, Pyrsocarpus, Jaundea, Pseudoconnarus, Rourea (Dickison 1973a).

Typology of foliar tracheoids in angiosperms 333

Fabaceae--Astrolobium (Heinricher Podalyrieae (tribe) (Sol. p. 294). p. 294).

1885), Genisteae (tribe), Loteae (tribe), Mimosaceae--ddenanthera, Piptadenia (Sol.

Myrtanae : Myrtales : Lythraceae~Crenea, Diplusodeon, Lawsonia (MC, p. 651). Rhizophoraceae-Bruguiera, Ceriops, Kandelia (So1. p. 341). Combre- tar (Sol. p. 918). Mela~tomataceae--Aciotis Belucia, Hen- riettea, Meriania (Sol. p. 921), Mouriri (Foster 1946), Memecylon (Rao 1957). Penaeaceae--Penaea, Saltera (-Sarcocolla) (Rao and Das 1976).

Saxiferaganae : Saxifragales : Feuquieriaceae--Fouqu&ria (Henrickson 1972; Larsten and Carvey 1974).

Primulanae: Plumbaginales : Limoniaceae~Limonium (Rao and Das 1968).

Plumbaginanae: Ebenales : Sapotaceae-Micropholis (Sol. p. 678).

Theanae : Theales : Theaceae--Visnea (Gramse 1907). Pelliceraceae--Pellicera (Sol. p. 121). Clusiaceae--Clusia, Endodesmia (Sol. p. 121). Nepenthaceae-- Glischrocolia (Pirwitz 1931), Nepenthes (Pant and Bhatnagar 1977).

Cornanae : Cornales : Alangiaceae--Alangium (Govindarajalu 1972).

Gentiananae : Gentianales : Menyanthaceae-Limnanthenum, Villarsia (Sol. p. 994).

Lamianae : Scrophularia!es: Scrophulariaceac--Anticharis, Antirrhinum, Euphrasia, Lindenbergia, Pedicularis, Rhinantheae (tribe) (Varghese 1969). Cesneriaceae-- Cyrtandra (Bokhari and Burtt 1970).

Caryophy!anae : Caryophyllales : Chenopodiaceae--Arthocnemum, Salicornia (Sol. p. 653; Pir~itz 1931).

2.2. Monocotyledons

Lilianae : Orchidales : Orchidaceae--Aerides, Coelogyne, Lipparis, Physosiphon, Pleurothallis, Vanda (Haberlandt 1914, Pirwitz 1931).

Aspergales : Agavaceae--Sansev&ria (Pirwitz 1931). Amaryllidaceae--Crinum (Pirwitz 1931).

An analysis o1 the aforesaid information indicates on a generous estimate that about 104 genera belonging to 45 families spread over 23 orders display tracheoids. Further, it is evident from the account under varied terms for tracheoids as proposed by the authors against each genus, there is an urgent need for further exploration to determine their typology and range of variations in their structural features in building up a satisthctory classification. The necessity for this type of study is illustrated in the findings of the re-examination of the so-called enlarged tracheids in Goupia (Solereder 1908) which are nothing but terminal vesiculose sclereids with wide lumina and thick sclerosed wall (Rao and Bhattacharya 1975).

Since the discovery of these curious cells based on their probable function as water storage element they have been designated as reservoir vasiformes (Vesque


1882), vascular cells (Gilburt 1881; Bierhorst and Zamura 1965), spiral cells (Schimper 1898), lignified idioblasts (Warming 1909), erweiterte SlCeichertrachciden (Sol0reder 1899), mechanical cells (Mangin 1882), storage tracheids (Haberlandt 1914; Sabnis 1921), water storage cells (Kny and Zimmermann 1885), or tracheoidal idioblasts (Foster 1956). Thus the correct designation of these idioblasts presents a problem in terminology. Tucker (1964) has given an admirable summary of the researches in this field. From Tucker's account, it is evident that there is a range in their cell form, structure and distribution pattern which can be utilised in building up a satisfactory typology. Furthermore, in her study one can see an attempt to segregate and identify the different types of tracheoidal elements occurring at the vein-endings in leaf expanses of many taxa of the Magnoliaceae. Tucker (1964) has recognised the following types based t~pon form and structure; conventional tracheids, dilated tracheids, and reticulate pitted cell-walled tracheids and also terminal sclereids including transitional elements. In the context of the possible usefulness of tracheoidal idioblasts as a diagnostic or taxonomic feature in plant s.vstematics an attempt has been made by Rao and Bhattacharya (unpublished), and Rao and Das (1976) to classify them into distinct categories. In this classification importance is given to the constancy of the body shape followed by important structural aspects which distinguish them from each other.

3. Classification

Depending on the constancy of their positional relationship with the vein.endings, the tracheoids exhibit two types of distribution--terminal and diffuse. Terminal tracheoids are mostly confined to vein-endings and rarely do they exhibit subterminal disposition. Diffuse tracheoids are dispersed in the mesophyll without any relationship to veins and vein-endings. The tracheoids irrespective of their pattern of distribution exhibit varied morphological diversity which could be used to build up a classification of some value for systematic purposes. Based on the constancy of cell form the tracheoids may be grouped into two kinds--homomorphic and heteromorphie. In the first, tracheoids have a simple cell form, smaller in size and show distinct structural or form variations to constitute an idioblast. Under the second category, tracheoids exhibit varied shapes and sizes, structural features and uneven outline.

3.1. Homomorphic tracheoidal types

The reeognised types under this group are brachytracheoids, sclerotracheoids, macrotrachooids, tubuliform spirotracheoids, vermiform tracheoids and loratotra- ohooids.

The salient morphological features of the above types are mentioned below. 3.1.1. Brachytracheoids (figures 5, 12, 19-21) : This type is represented by

relatively simple base forms. Usually it is spheroidal, ovoid, globoid or orbiculate, ellipsoidal or curiously dilated with knobs. They are either diffuse or terminal to the vein-endings in the leaf expanses. Despite variation in shape, they are smaller in size in the laminae of several taxa. Their presence along with the conventional tracheids is rather common among the tropical species.


Figures 1-15. Somidiagrammatic sketches from different taxa showing main types and deviations in the formation of trachooids. 1. Conventional tracheoids. 2. Lorato- tracheoid. 3. Tubular spirotracheoid. 4. Diffuse tubular tracheoid. 5. Brachy- trachcoid. 6. Tracheoids-in=aggregates. 7-9. Macrotracheoids. 10. Hcterotrachooids. 11. Vermiform tracheoids. 12. Brachytrach~oid. 13, Sclerotracheoid. 14. H~brid ~:ell, 15. Sele.reid,


Under this category, three forms can be recognised based on structural variations. The first corresponds to the description of dilated tracheids sensu Tucker (1964) oharaoterised by differential wall thickenings as annulae or helices and it is exempli- fied in a few taxa of Liroidendron, Magnolia (Tucker 1964), Connaraceae (Dickison 1973a) and also in a few species Boronia (Rao and Bhattacharya 1978), Capparis and Hibbertia (Rao and Das 1978, 1979b). In the second, the wall is relatively thin showing annulae or helices all over the wall and are recorded in magnoliaceous leaves (Tucker 1964) and also in a few species of Boronia (Rao and Bhattaehalya 1978) and Capparis (Rao and Das 1978). In the third, the cell-wall is relatively thin showing reticulate pits. Many taxa from Magnoliaceae (Tucker 1964), Connaraceae (Dickison 1973a) and Azima tetracantha Lam. of Salvadoraceae (Govindarajalu and Parameswaran 1967) have been referred to possess this type of traoheoid at the veinlet endings. This type has also been observed in Capparis (Rao and Das 1978).

Figures 16-18. Semidiagrammatic sketches of the cleared laminae. 16. Pogono- phora schomburgkiana Miers ex Benth. var. elliptica Pax. showing diffuse gymnomacrotracheoids in rows. 17-18. Xanthophyllum species. Branching trend in gymnomacrotracheoids (After Dickison 1973).


The brachytracheoids show many features of the conventional traoheids despite their variability in size and shape. Apparently they seem enlarged discrete entities associated with and in continuation of the normal tracheary elements. They are found as solitary or pairs of idioblasts or in multiples in the form of concretions around the vein-endings. Such cell aggregates have been recorded in Platymitium loranthifolium Warb. and Dobera roxburghii Planch. of Salvadoraceae (Solereder 1908). Similar encystment of thin-walled pitted elements around vein-endings has been reported (Sabnis 1921) ill Salvadora oleoides Decne and S. persiea L. Also thick-walled simple pitted elements around vein-endings are recorded in Dobera loranthifolia under the type agglomerate ~lereids (Govinda- rajalu and Parameswaran 1967). It is to be noted here that thin-walled or thick- walled cell aggregates with orderly pitting constitute tracheoids in aggregates rather than sclereid aggregation or agglomerate sclereids. This can be further illustrated by such cell aggregations as found at the veinlet endings in Capparis. Under the traoheoids-in-aggregates one can recognise two types of cell variations based on the thickness of cell-wall. In the first, tracheoids have a thin wall, wide lumen and orderly simple pitting. In the second, the wall is thick, often striated, with wide lumen and orderly simple pitting.

Small-sized braehytrache~oids or tracheoidal nodules are found in abundance at the vicinity of veins or vein-endings in the areoles, at the apices, and along the margins of leaves. This feature is observed in a few Euphorbia (Sehgal and Paliwal 1974), Boronia (Rao and Bhattacharya 1978) and also ill several Scrophu- lariaceae (Varghese 1969).

Similar but sclereified tracheoid nodules associated with minor veins and vein- endings in the distal part of Fouquieria leaves have been designated as water storage tracheoids (Henrickson 1972). These cells are called by the neutral term veinlet elements instead of tracheoids or sclereids because the secondary wall of some cells has only sparsely distributed simple pits, whereas others have boardered pits (Larsten and Carvey 1974). A mixed association of thin-walled brachytracheoids and sclerotracheoids has been observed in the laminae of a few Boronia (Rao and Bhattacharya 1978).

3.1.2. Sclerotracheoid (figures 13, 19-21) : This category includes idioblasts which resemble brachytracheoids in their spirally thickened or pitted walls but differ from them in possessing thick sclerosed cell-wall and relatively bigger cell form. In the present study they are illustrated in a few species of Capparis (figures 19-21). Their occurrence along with brachytracheoids or conventional tracheids is rather restricted to ameng the tropical species. In their diffuse distribution they correspond to liguified idioblasts defined by Warming (1909).

In this category, there are distinct peculiar cell forms known under various terms like transitional cells (Foster 1946), hybrid cells (Rao 1957), terminal tracheoid idioblasts (Foster 1956; Rao and Mody 1961), intermediary cell forms (Dickison 1970), half-way house between the normal trach~ds on the one hand and the full- fledged sclereid on the other (Govindarajalu and Parameswaran 1961) and veinlet elements (Larsten and Carvey 1974). 'They are observed at the vein-endings in such genera as Mouriri (Foster 1946)and Memecylon (Rao 1957) of the Melastomataceae, Ptychopetalum (Foster 1947), and Olax of the Olacaceae (Rao and Mody 1961), a few taxa of Connaraceae (Dickison 19"~3a) and Hannoa of the Simaroubaceae (Foster 1956). The hybrid character of these cells in resembling sclereids in their

338 T Ananda Rao and Silpi Dos

sclerosed, lobed cell-wall and tracheids in possessing helical wall thickening or orderly pits needs thorough morphogenic studies which perhaps may solve the morphological identity of these cells and also help in devising correct nomenclature for tracheoid idioblasts or sclereids.

3.1.3. Macrotracheoids (figures 7-9, 16-18, 22-25) : This term is proposed by Rue and Bhattacharya (in press ) to big-sized tracheoids mentioned in the literature (Solereder 1908; Metcalfe and Chalk 1950). They may diffuse as in the lamina ofPogonophora schomburgkiana Miers ex Benth. (Foster 1956) and its two varieties (present study) var. elliptica Pax. (South America, Rio Janerio, Glasion 15419, LE) and var. longifolia Pax. (South America, Brazil, Rio Janeric, R Spruce 1996, LE) of the Euphorbiaceae (figure 16). These cells are ovoid ellJpsoidal, big-sized traoheoid-like idioblasts which are dispersed throughout the mesophyll of certain orGhids, Sansevieria, Crinum and in many dicotyledonous genera. It is evident from the taxa listed that on many tracheoids of different genera detailed accounts are not available. In recent years, details on such comparable idioblasts are available in a few species of Salicornia (Fraine 1912; Falm and Arzee 1959), in many species of Xanthophyllum (Polygalaceae)(Dickison 1973b), in the mesophyll of Cyrtandra of the Gesneriaceae (Bokhari and Burtt 1970) and in the mesophyll of Vochysia of the Voehysiaceae (Rao and Mody 1961). Based on structural features 2 types of macrotracheoids are recognised : angiomacrotracheoids or sheathed macrotrachooids and gymnomacrotracheoids or naked macrotracheoids. Unlike tee gymnomaorotracheoids of PogOnophora and its varieties (figure 16), the big ellipsoidal sized angiomacrotracheoids in the laminae of Xanthophylhtm show distinct terminal or subterminal position in relation to vein-endings and also a tendency to branch (figures 17-18; 22-25). Their terminal attachment is very conspicuous in the cleared laminae of many species of this genus. The macrotracheoids of Cyrtandra are usually larger in size, and dispersed in the palisade tissue in a bowl- like depression (Bokhari and Burtt 1970).

Structurally these macrotraeheoids have spiral thickenings which are very closely spaced in Xanthophyllum. In Cyrtandra t h ~ have annular thickenings or pitted walls (figure 9).

3.1.4. Tubuliform spirotracheoids (figures 4, 28): These cells are mostly tubular, empty and their walls are stiffened by a spiral band, composed of three or more partial bands. This is illustrated in the laminae of Nepenthes (Solereder 1908; Pant and Bhatnagar 1977).

3.1.5. Vermiform tracheoids (figure 11) : The term vermiform parenchyma was introduced by Govindarajalu (1972) to the vein-endings which possess finger- like projections or to proliferated, elongated stnlctures extending considerably beyond the limits of the vein-endings thereby making inroads within tl~e mesophyll tissue. They are unicellular or multicellular without contents, with worm-like appearance showing many contortions or convolutions, and often with simple pits. They are illustrated in the laminae of Alangium (Govindarajalu 1972). It would be better to redesignate such forms as terminal vermiform tracheoids in view of their idioblastic character from the surrounding organised parenchymatous tissue. Further, it is clear that the arguments advanced to keep the two terms. Enlarged terminal tracheary element and tracheoidal idioblasts for the same structure which are structurally alike but positionally different is not justified.


Figures 19--21. Cleared laminae of Capparis aegyptica "Lamk. showing brachy- traeheoids and sclerotracheoids ; note the differences in wall thickening, • 1200 each.


Figures 22-25. Cleared laminae. 22. Xanthophyllum eurluynchum Miq. showing terminal Gymnomacrotracheoids; note absence of sheath cells, • 600. 23-25. Xanthophyllum scorteehinii King showing angiomacrotracheoids. 23. Remains of sheath cells at the outline of tracheoids. 24-25. Broken sheath cells present, • 1200 each.


Figures 26-28. 26. Memecylon barteriHook, f. showing brachytracheoid, sclero. traeheoid and sclereid in row, • 1200 (After Rao T A and Jacques-Felix, H 1978). 27. Memecylon guineense Keay--Twin occurrence of sclerotracheoid and sclereid, • 532 (After Rao, T A and Jacques-Felix, H 1978). 28. Nepenthes ampullaria Jack--Tubular spirotracheoid, • 150 (After Pant D D and Bhat- nagar S 1977).


At best they may be considered as terminal or diffuse in their position to the vein- endings.

3.1.6. Loratotracheoids (figure 2) : Rao and Das (1976, figure 3) proposed this term for the wandering tracheoidal veinlet elements which are strap-shaped, relatively thin-walled, helically thickened and possess uneven width and rounded ends. They are nothing but a continuation of the tracheoidal veinlet often found traversing the spongy or palisade region of the mesophyll, especially near the marginal areas in the lamina of Penaea mucronata L., Sarcocolhl acuta Kth. and S.fureata Endl. of the Penaeaceae.

3.2. Heteromorphie traeheoidal types

Under this category are included tracheoids of varied morphological diversity especially in respect of size, shape and position in the laminae. Their relationship is either terminal or diffuse to the vein-endings.

3.2.1. Heterotracheoids: This term is proposed by Rao and Das (1976) to tracheoids of varied shape and size recorded at the vein-endings in the laminae of Penaea encorum Meerb (Penaeaceae). The inclusion of this subtype under idio- tracheoids sensu Rao and Das (1976) seems to be superfluous and the term heterotracheoids is considered appropriate and explicit from a morphologk.al viewpoint.

These are found in loose clusters at the veinlet endings and resemble macrotracheoids but differ from them in having uneven wider diameter leading to diverse shapes and sizes (figure 10). Usually they are thin-walled with spiral thickenings. They have be~n observed in the laminae of Saeroeolla retzoides Eckl. and Zeyh.

4. Conclusions

Tracheoids in angiosperms are not only of morphological interest but may also help in various ways in taxonomy if they are spread as a generic or group character in relation to a particular taxon. In certain species they do serve as a diagnostic feature. Dickison (1973b) working with Xanthophyllum (Xanthophyllaceae-- Polygalaceae) provides an instance of taxonomic use of tracheoids. A perusal of scattered information contained in Solereder (1908), Metcalfe and Chalk (1957) and review of their occurrence in a few taxa by Pirwitz (1931) suggest the potentia- lities of these structures in future taxc.nomic studies. Tracheoids, however, are most often not enlisted for systematic comparison because of lack of comparative studies on them.

Depending on the shape, size and structural features, seven principal types of traeheoids have been reeognised. These types characterise certain families, genera or even species. Sometimes a tew types show intergradation within a single leaf.

A perusal of the tracheoids as proposed in the present study shows certain series or deviations of morphological interest (figures 1-15, 26-28). Firstly, starting from the simple conventional tracheid, the deviation is towards formation of loratotracheoids or tubuliform spirotracheoids. Secondly, brachytracheoids represent the next type of deviation over the conventional tracheids; these, in turn, show distinct entities namely macrotracheoids in two forms: gymno-


m a c r o t r a c h e o i d - - n a k e d ; a n g i o m a c r o t r a c h e o i d - - s h e a t h e d , v e r m i f o r m t r a c h e o i d s

a n d he t e ro t r acheo id s . A n o t h e r d i s t inc t dev ia t ion is t o w a r d s f o r m a t i o n o f t r acheo ids - in -aggrega tes . Las t ly , the occur rence o f b rachy t racheo id% sclero-

t racheoids , hyb r id cells and sclereids within a single leaf cons t i tu te a d is t inc t series o f exGeptional interest (Rao and Das 1979a). One can conc lude such a t rans- f o r m a t i o n as the sclereid to represen t a specia l i sa t ion over the t racheoids . Such a conclus ion , however , should be conf i rmed by on togen ic s tudies because o f the i m p o r t a n c e o f such in fo rmat ion .

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Typology of foliar tracheoids in angiosperms

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