The common, or small-leaved lime (linden tree, basswood) (Tilia cordata P.Miller; Tiliaceae) displays surprisingly much variability. Some of this variability is well-known, some has been noted one time but has essentially remained neglected and escaped common knowledge, while some features or forms have probably been not noted at all in the literature. A good part of this variability seems to be genetically determined, and some traits might even be worth of a deeper analysis to find out if they are taxonomically significant. Some variation may be peculiar to single trees or clones alone but a part of the diversity may even be discernible at the of population level. On this web page I present some observations of mine made during years in the Finnish woods.
Characters that, in particular, display apparent hereditary variability
Degree how pendulous, or hanging the twigs are.
Size of leaves. – Besides "normal" variation, a rare form with strikingly small leaves with a rather truncate base to the lamina has been found in Finland: T. cordata f. betulifolia (Bayer) V.Engler.
General shape of leaf blade. – (i) The latter is mostly more or less cordate, rarely rather ovate with almost truncate base (see preceding section)). (ii) how long the apex is attenuated. (iii) Relation length : width of the leaf lamina.
Width of the basal sinus of the leaves of a short twig. – In a comparison between the leaves of different trees, it is important that leaves in a homologous position are compared: the basal sinuses of the successive leaves (usually 3) in the twig (the short shoot of current year) get wider acropetally, i.e., the basal leaf has the narrowest sinus and the apical one the widest sinus. This appears to be a strict rule in T. cordata but this feature is but very seldom mentioned in the floras (Fig. 1 below). – T. cordata var. asymmetrica Borbás and T. cordata f. betulifolia are at one end of variability in this character, possessing wide sinuses with that of the uppermost leaf even being roughly 180º, i.e. the base of lamina is straight (sometimes it is in oblique position so as to result in an asymmetrical blade). The tree in Fig. 1 represents the other extreme of the variability in this feature: the basis of lamina is peltate in the 1-2 lowermost leaves.
Margin of leaf blade. – Typically it is non-lobed, only serrate. A deviating form with shallowly 3-lobed leaves is very infrequent: f. vitifolia (Wierzb.) V.Engler. Some trees of this kind have have been found in Finland, such as that represented in Fig. 2 (below).
Thickness of the leaves. – The leaves appear to be thickest in well-lit habitats and twigs but possibly also genetic variability exists.
Amount of wax on underside of leaves. – This variability results at least partly from environmental factors as the thickness of wax covering in the leaves seems positively correlated with the amount of light available to the tree or a given twig). Typically, the underside is glaucous green and dull due to a thick wax layer. More rarely, the wax amount is small; in cases when there is seemingly no wax at all, the leaf underside is pure green and more or less shining. The name T. cordata f. viridis auct.(?) (onko laill. nimi? ei löyt. Hegistä) has been given for the latter kind of trees (Fig. 1 below).
Intensity of the yellow autumn colour of the leaves. – Some trees assume bright yellow colouration towards the autumn while others have some brownish tinge intermixed; the latter also seem to get more rapidly truly brown after having shed onto the ground.
Response of the leaves to UV radiation. – Accidentally, I observed an interesting response to daylight of leaves picked in summer and left on table by a window. Leaves from some trees gradually became dull brown after a few months while those from other trees other remained predominantly green for almost a year. Later, the phenomenon was checked intentionally with leaves of different origins, and such differences were found repeatedly. No doubt the ultra-violet radiation present in daylight causes the browning (even if the light is diffuse, no direct sunshine was allowed). This variability may be related to that treated in the preceding section. Differences in the amount and kind of epicuticular wax may be the ultimate reasons for different responses.
Presence of simple, hyaline hairs (like those found in T. platyphyllos Scop.). – Usually such hairs (unlike some other kinds of hairs) they are totally absent (excepting the leaves and even twigs of some young individuals and rapidly grown basal shoots). At times, however, scattered simple, hyaline hairs are found on the undersides and at the margins of the leaves of larger trees; it remains to be cleared up whether their presence is genetically determined at least in some cases.
Number of flowers in the inflorescence.
Shape of fruit. – It may be broadly fusiform, ellipsoid or globose.
Fig. 1. Tilia cordata, forma (Finland, Lohja, Vanhakylä, 2000). – Image: Harri Harmaja (scanned from dried specimen). – Note that: (i) As a rule in lime tree the basal sinuses of successive leaves in a twig (the yearly short shoot) get wider acropetally: the basal leaf has the narrowest sinus and the uppermost leaf the widest sinus. (ii) The tree figured is unique in that this character is expressed in a "exaggerated" way as the lowest leaf (rarely two lowest leaves) of the twig has a peltate base, or the adjunction point of the petiole and the blade! In some twigs of this same tree, however, the lowest leaf is not peltate, only having a very narrow sinus basally. The expression of this character thus somewhat varies with twigs and possibly also with growing seasons. Anyway, this tendency of the lowest leaf appears to be genetically determined and would thus deserve to be recognized as a taxon of its own at the level of forma. (iii) At the same time, this tree belongs to those who lack a visible wax layer on the leaf underside, the latter being pale green and more or less shining instead of glaucous and dull.
Fig. 2. Tilia cordata f. vitifolia (Wierzb.) V.Engler (Finland, Lohja, Torhola, 1995). – Image: Harri Harmaja (scanned from dried specimen. – These freshly shed leaves were picked from the ground below a slender but high lime tree. Actually, this was the the only possibility to detect this form with shallowly lobed leaves because all its twigs lie very high!
Created March 17, 2004.