Taxonomic significance of vessel elements in the Tiliaceae

In ternodal vessel elements were investigated in 7 genera and 27 species of the Tiliaceae with special reference to their taxonomic significance. On average, the longest vessel elements are 441 )..1m , recorded in Grawia asiatica and Triumfetta rhomboidea, and the shortest 78 f.lIT1, in Grewia microcos. The average maximum diameter is 79 j..1m in Corchorus aestuans, and the minimum 22 j..1m in Grewia microaos. The vessel elements are Bither cylindrical, spindleshaped, column-, drumor funnel-like, conical , clavate or oval. Perforation plates are simple in all the species studied, except in Grewia asiatica , where ephedroid-like perforation plates were recorded. There are usually two perforation plates in the majority of the members, rarely three or one. Adjacent wall thickening is commonly simple-pitted, but bordered, reticulate or scalariform pits have also been observed. Vessel elements vary in form and structure, number and inclination of perforation plates and side wall thickening. These characters are of taxonomic importance.


Introduction
Vessel elements have been studied extensively in different families of monocotyledons by Cheadle (1943Cheadle ( , 1944Cheadle ( , 1953Cheadle ( , 1955Cheadle ( , 1968 and Cheadle and Kosakai (1972, 1973, 1974, 1975, 1980. Very liltle attempt has, however. been made to study the diversity of vessel elemenls in the dicotyledons (Bailey & Tupper 1918;Bailey 1944; Abbe & Abbe 1971; Inamdar & Murthy 1977;Murthy er al. 1978; Alekutty & Inamdar 1978;Shenoy & Inamdar 1979; Avita & Inamdar 1981;Inamdar et al. 1983Inamdar et al. , 1985Bhat er al. 1989Bhat er al. , 1992.) Bailey (1957) has pointed out the taxonomic imporlance of the sludy of xylem elements, especially the vessel members of various genera and species of lhe angiosperms. A literature review revealed that lhe vessel elements of Tiliaceae have not yet been s tudied. The present study describes diversity of the internodal vessel elements in 7 genera and 27 species of the Tiliaceae, with a view to drawing some conclusions about taxonomic implications of the assembled data.

Material and Met hods
Plant material was collected from different parts of India. Mature internodes of fresb material, fixed in FAA, and herbarium material were macerated following the method of Jane (1956). Macerated tissue was washed tboroughly in distilled water, stained in Delafield's haematoxylin and mounted in glycerin jelly. All observations are supported by diagrams, drawn by using camera lucida at constant magnification (Figures 1-70).
The size of the vessel elemenlS represenlS mean values of 30 readings. The list of species investigated, size of vessel elemenlS, disposition and number of perforation plates and type of adjacent wall pitting are compiled in Table 1, arranged in alphabetical order.

Size
Internodal vessel elements show grea t variation length and are therefore, for the sake of convenience, grouped into long (> 345 I'm), medium (205-344 )lIl1) and short « 205 I'm) (see Table 1).

(b) Medium
Average length of medium vessel elements ranges from 312 ~m in Corchorus [ascicuLaris to 207 J.lm in Triumfetla pilosa, with diameter varying from 95 !-lm in Triumfetta rhomboidea to 22 llm in Grewia microcos.

(e) Short
Average length of short vessel elements ranges from 153 !-lm in Grewia lenax to 78 jlm in Grewia microcos, and the diameter from 59 !-lm in Grewia villosa to 24 J.lm in Tricuspidaria dependens.

Adjacent wall thickening
The side wall thickening may be simple-pitted, border~pitted, reticulate or scalarifonn. Simple pits are circular or oval ( Figures  1-38 , 40-53, 55-70), and their arrangement may be opposite, alternate or random . The bordered pits vary in sbape and arrangement (Figures 39, 54). They may be transversely elongated in a ladder-like fashion (scalariform) or arranged in diagonal rows or lines (alternate) (Figures 39, 54). Simple-pitted side wall thickening is, however, commonly observed in Tiliaceae. Details of the vessels of the various specimens are given in Table I.

Discussion
The Tiliaceae comprises 50 genera and about 450 species widely distributed in tropical and temperate regions. but chiefly in South East Asia and Brazil (Willis 1973). According to Metcalfe and Chalk (1950), vessel elements in Tiliaceae are small· to mediumsized, with simple perforation plates and in tervascular pitting minute to mOderately large. The longest vessel elements recorded during the present study are 441 ~m in Grewia asiatica, and the shortest 78 ~m in Grewia microcos. Radford er at. (1974) classified the vessel elements taking length as a criterion. In our investigation the vessel elements fell in the categories extremely short « 175 ).LID) to medium-sized (350-850 ~m). The maximum and minimum diameters ranged from 95 to 22 ~m in Trium/elta rhomboidea and Grewia microcos, respectively. The diameters of the vessel elements also fell under the categories of extremely small « 25 )J.m) to moderately small (50-100 I'm) of Radford el al. (1974). Inamdar el al. (1983) and Bhat el af. (1992) made similar observations in the members of Malvaceae and Sterculiaceae, respectively. These investigations, therefore, conform with those of Metcalfe and Chalk (1950). The shapes of the vessels were cylindrical, fusiform, column-like, conical, fish-like, cup· like, clavate, oval, funnel-like or erratic. The majority of the vessels were, however, cylindrical. Perforation plates were exclusively simple in all the species investigated. The disposition of the perforation plates was mostly oblique or lateral and occasionally median and transverse. The number of perforation plates was mostly two, but there were vessel elements with one or three perforation plates among the species studied. Side wall thickening in most species was simple-pitted with opposite, alternate or random arrangement, but bordered, reticul ate, mixed bordered with simple·pitted or scalariform types were also observed. The vessel members of the family were mostly without tails. Vessel elements closely resembling tracheids are considered most primitive and those least resembling them most specialized (Cheadle & Kosakai 1980). Short vessels with an end wall transversely placed and having a single, simple perforation plate are also considered to be the most specialized statc (Cheadle & Kosakai 1982).
Since most of the features regarding vessel characters are common among the tribes of the family, it can be concluded that the family Tiliaceae comprises a natural group of tribes. Vessels in the Tiliaceae are highly specialized in the internodes. and the present investigation revealed many advanced features, such as the short vessels, with simple perforation plates at the transverse end and simple pits with alternate arrangement.