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Limes, i.e. trees of the genus Tilia, are unmistakable when in flower or in fruit (provided the flowers/fruit are not borne so high up the tree that they cannot be clearly seen).
The inflorescence is a branched cluster of flowers. (Technically a panicle.) To the common stalk (the peduncle) of this cluster is attached a usually oblong, green, yellow or white bract, whose midrib is fused to the peduncle for about half the length of the midrib. This bract morphology is unique amongst any trees that one is likely to see within the British Isles, and is, I believe, unique among all plants. This bract is retained by the infructescence, which is therefore also distinctive. The flowers are small (typically ½" in diameter) and are whitish or yellowish. The fruits are small nuts, which are spheroids, ellipsoids or ovoids.
[ I have seen variations in the bract morphology of individual inflorescences, but the great majority of inflorescences on any given tree are as above. Variations include the presence of a pair of unfused auxiliary bracts above the main bract, the bract being absent, the bract not being fused to the peduncle, and the bract being unfused, orbicular and auriculate. ]
Even when not in flower or fruit, i.e. in the spring, or when a tree is too young to flower, or is not flowering in a particular year, limes can fairly readily be recognised by their foliage. The leaves are distichous (borne in two rows), alternate (borne one to a node, alternately on either side of the twig), petiolate (stalked), stipulate (with a pair of small leaf-like structures at the base of the leaf stalk - but these fall off early in limes and are not always to be seen), simple (not divided into leaflets), unlobed, palmately veined (multiple veins radiating from the junction of stalk and leaf-blade), cordate (heart shaped) or truncate (leaf margin perpendicular to the petiole) at the base, often obliquely so, and acute to acuminate (pointed at the apex), borne on zigzagged young shoots. This combination of characters is rare amongst temperate trees; even the combination of unlobed leaves and palmate venation is rare.
The trees which, in the absence of flowers or fruit, might be hardest to distinguish from a lime are some rare maples, such as the Birch-leaved and Lime-leaved Maples. Commoner trees which can at first glance be confused with limes are elms (Ulmus), poplars (Populus), beeches (Fagus), alders (Alnus) and some whitebeams such as Sorbus aria and Sorbus cuspidata. In all cases the flowers and fruit are immediately distinguishable from those of limes.
The leaves of elms (Ulmus) are most similar in general appearance but are asymmetrically pinnately veined, and generally more elongated than those of limes. The leaves of those populars which are similar to those of limes (i.e. excluding the aspens and the white poplar) are usually no more than 3-veined at the base. Those of beeches (Fagus) are pinnately veined, and are also cuneate (leaf margin meets the petiole at a diverging acute angle) at the base. (I don't know why I find that beeches have a similar 'jizz' to limes - you might not find them confusable - but the leaves of limes and beeches are utterly distinct, so they are easily distinguished when close enough to see the leaf shape.) The leaves of alders (Alnus) and whitebeams (some Sorbus) are also pinnately veined.
Rarer trees which might be confused with limes are the mulberries (Morus), katsura (Cercidophyllum) and handkerchief tree (Davidia). The mulberries are typically 3-nerved at the base; the upper surface of the leaves of the Black Mulbery (Morus nigra) have a sandpaper-like texture to the touch. The katsura is palmately veined. However its leaves are opposite (borne in pairs) and have a blueish cast on both sides, which serves to distinguish them from limes. The leaves of the handkerchief tree are pinnately veined.
Many maples have palmately veined leaves, but it seems that those species with unlobed leaves similar to those of limes are typically 3-nerved at the base.
Although identification of a tree as belonging to genus Tilia is easy, identification of species within this genus is not. (Some books will say that identification of this or that lime species is easy; this is not the case if you haven't got your eye in.) The foliage varies between individuals of a species, and even on a single tree.
[ When identifying limes ignore strongly growing stems at the base of the trunk; the foliage of these stems is atypical of the plants. Whether or not it is distinguishable as to species, it is not concordant with descriptions of the normal foliage of the species. Pleached limes are difficult to identify for the same reason. ]
Five types of lime are commonly planted in Britain. These are Tilia cordata (Small-leaved Lime), Tilia ×vulgaris (Common Lime), Tilia platyphyllos (Large-leaved Lime), Tilia tomentosa (Silver Lime) and Tilia ×euchlora (often called Crimean or Caucasian Lime). Other types of Lime are mostly confined to collections (e.g. RBG Kew, Westonbirt, Thorp Perrow, RBGE Dawyck, RBGE Leith). For these the easiest means of identification is to look at the label.
[ The teeth of the leaves of Tilia henryana are filamentous. This might be sufficient to identify this species, but some forms of Tilia americana are described as bearing awn-shaped teeth. ]
Tilia ×vulgaris is a hybrid between Tilia platyphyllos and Tilia cordata. This includes backcrosses to the parent species, and seedlings from T. ×vulgaris, which may approach either of the parent species, especially T. platyphyllos. I am unconvinced that there are morphological breaks between these three taxa, and hence that all trees can be correctly identified. However over 95%, and perhaps 99% of trees are identifiable, provided a view can be obtained of the leaves and inflorescences.
Tilia cordata: The Small-leaved Lime is readily identified when in flower. There are two forms. In one the flower clusters are held upright; in the other they are held at various ascending and descending angles. In either case this serves to distinguish it from the other common limes, in which the clusters are pendulous. (Both forms are also shown by various east Asian relatives of this species, including Tilia insularis and the Japanese Lime, Tilia japonica.) Otherwise it may be recognised by the small neatly cordate leaves, often distinctly triangular in form, which often, but not universally, develop a blueish cast on the underside by July, and which have brown or orangish tufts of hairs in the axils of the leaf-veins. The bluish cast is found rarely in trees otherwise agreeing with T. ×vulgaris, and I have seen it in one specimen of T. ×euchlora. The brown/orangish axillary tufts are shared with perhaps half the specimens of T. ×vulgaris.
Tilia cordata flowers later than T. platyphyllos and T. ×vulgaris, in July. The flowers are whitish.
I have seen a single specimen of T. japonica, which has smaller leaves than T. cordata, and lacked a bluish cast to the underside of the leaves. I hesitate to suggest that this would reliably distinguish this species from T. cordata, never mind from the other Asian small-leaved Limes.
Tilia platyphyllos: The Large-leaved Lime is the earliest Lime in flower, in early to mid-June, and generally has fewer and larger flowers and fruits than T. ×vulgaris (and T. cordata). The typical inflorescence has 3 flowers, but it is said in the literature to vary up to 5, and I have seen plants with 6-flowered inflorescences otherwise agreeing with T. platyphyllos.
The size of the leaf is not a reliable guide, it usually not being appreciably larger than that of T. ×vulgaris, and sometimes quite small. The foliage, especially when young, can be distinguished by its pubescent nature. The upper surface of the leaf is pubescent, especially early in the year, as are the twigs and leaf-stalks. The pubescence of the upper side of the leaf is not always visible to the naked eye, but the hairs can be seen when looking obliquely at the leaf blade in strong sunlight; however it is often appreciable to the touch. The underside of the leaf has tufts of white hairs in the axils of the leaf veins, and additional hairs along the veins. The latter is not always visible without magnification. The leaves are typically darker and rougher than in T. ×vulgaris.
The degree of hairiness varies in the wild, trees from more southerly populations being less hairy. Where trees of southerly provenance have been planted in Britain they might be confused with T. ×vulgaris.
Tilia ×vulgaris: The Common Lime can be recognised negatively by not being any of other Limes. However the leaves are glabrous (hairless), except for tufts of hairs in the axils of the veins, and rarely a few along the veins. These tufts may be white, as in T. platyphyllos, or brown, as in T. vulgaris. The inflorescences are pendulous, and are borne later than T. platyphyllos, in late June, but overlap in time, and usually have more flowers, the flowers and subsequent fruits being typically smaller. All of T. platyphyllos, T. ×vulgaris and T. cordata can produce some sprout shoots from the trunk, particularly at the base, but some forms of T. vulgaris carry this to extremes, producing masses of shoots from burrs at the base of the trunk, and sometimes even over most of the height of the bole. Hence any tree with particular dense sprouts may be presumed to be T. ×vulgaris.
Tilia tomentosa: The easiest lime to identify is the Silver Lime, Tilia tomentosa. Large specimens are infrequent, but the species is now commonly planted and plantings of small groups and individuals are frequently encountered. This has broadly ovate leaves which are whitish beneath, and lack tufts of hairs in the axils of the leaf veins. The young twigs and leaf-stalks also have a dense white coating of fine hairs. It is the latest flowering of the commonly planted Limes, flowering in late July and early August. The flowers are strongly scented, and have a more distinct yellow cast that the other types.
Its cultivar 'Petiolaris' is about equally common. Is is distinguished from the normal form by its pendulous habit, long leaf-stalks (over half the length of the blade of the leaf, whereas in the normal they are usually, but not universally, less than half the length of the blade) and depressed globose (oblately spherical) fruits. This form is nearly sterile, and the differing shape of the fruits might be a consequence of this.
The rare Oliver's Lime (Tilia oliveri), from China, differs in having glabrous (hairless) twigs and leaf-stalks, in having tuberculate fruit, and in flowering in June, rather than late July and early August. Tilia maximowicziana, from Japan, has tufts of brownish hairs in the axils of the leaf veins. Some forms of the American Lime (Tilia americana) also have whitish leaf undersides, but their leaves are less rounded.
Tilia ×euchlora: This is commonly known as the Crimean or Caucasian Lime, and is believed to be a hybrid between T. cordata and T. dasystyla. It is now frequently planted as a park or street tree, as it doesn't suffer from the attacks of aphids to the same degree as other species. This is unmistakable when in fruit. The fruits are ovoid, i.e. narrowed away from the stalk, whereas in other limes they are globose, ellipsoid or obovoid (narrowed towards the stalk). Otherwise this hybrid can be recognised by the dark green shiny leaves, paler underneath, and the clear green twigs and leaf-stalks. (I have seen several atypical specimens, one with yellowish-green twigs aging to orange, and a few with a bluish cast to the underside of the leaves.)
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© 2003, 2004 Stewart R. Hinsley