The Laburnum Tree { Study of Trees } 20

Laburnum in flower is a beautiful tree

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Introduction

Laburnum is the twentieth article in the 'Study of Trees' series, where the intention is to help those who are interested in trees but are unsure of the species. With the aid of descriptive text and images the article will make the process of identification easier. This will be accompanied by background and historical information.

All tree have a distinctive character{s} that will help the observant to recognise the species. here we look at the Laburnum which has two principal species and other close allies.

About the Laburnum

Laburnums belong to the Order of trees known as the Fabiales and the family Fabiaceae within that order. they are further placed in the tribe Genistecae. There are two distinct species of Laburnhum, the common Laburnum, Laburnum anagyoides, and the Alpine Laburnum, Laburnum alpinum. In days gone by Miller recognized the two but Linnaeus did not. Today they are indeed recognized as two distinct species.

As for an example Quercus rober the English oak and Quercus petrae the Sessile oak come true from seed so do the two species of Laburnum. Both sorts are very ornamental and have been extensively propagated and cultivated in British gardens for centuries. Records reveal there were trees at Syon { Syon House with its Parks West London} of L alpinus above forty feet high and some at Alniwick Castle { Northumberland -north east England} with trunks three feet in diameter.

Flowers of the Common Laburnum 'Golden rain'

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General and historical information about the Laburnum.

Hares and rabbits were believed to have been fond of the bark of Laburnum,and it was suggested by certain authorities to sow Laburnum seeds, in order to produce an undergrowth in plantations likely to be infested with these animals. " For though the plants are eaten to the ground every winter, yet they will spring up again the next season and thus yield a regular supply of winter's feed for these types of game."

As an ornamental tree the Laburnum has few rivals. Though it will grow in very different soils ,it requires a deep fertile sandy loam to attain a large size. In regard to situation, as the tree puts out a few horizontal roots and has a rather spreading canopy,it is apt to be blown aside by high winds.,but for the same reason ,it is less injurious to plants growing near it. Conversely for the same reason it is less injurious to plants growing near it than some other ornamental trees.

The common Laburnum was formerly named Cytissus laburnum. it is a native of mountainous woods in, central Europe. However, it has been long been cultivated throughout Europe. There does not seem to have any existing record of when the tree was introduced into Britain.

The origin of the name Laburnum,seems to have been lost in time. One put forward by Dr.Prior seems to be very fanciful, he suggested it was an adjective from the Latin Labor, denoting what belongs to the hour of labour,and which supposedly alluded to the leaflets closing together at night and expanding by day.

Although the name of Laburnum seems a little obscure it has made up for it with a multitude of common names both here in the UK and on the Continent of Europe. In Scotland the tree's blossoms have gained it the name of Pea tree,and 'He broom', the latter probably meant high broom as opposed to the Common Broom** and the low broom Genista tinctoria. In Lincolnshire it was referred to as 'Golden drops',and in other localities Golden rain and Golden Chain.

** This species has already been reviewed in this series.

Description

The shape of the tree.

The tree's canopy is irregular but picturesque. Its foliage is smooth green, and shining,and it is not vulnerable to be preyed upon b y insects. It attains the height generally of twenty to thirty feet retaining a smooth, grey-green rind on which the 'tenticles' or cork-warts are somewhat conspicuous.

A thin paper-like epiderm is , in fact,sloughed off by the shoots at an early stage,so that the permanent surface is a secondary periderm. As it is not a fast growing tree, its wood is dense and often shows very regular concentric growth, the few outer rings of yellow sapwood sharply contrasting with the heart wood.

The heart wood of Laburnum is of a dark colour and though of a rather coarse grain, it is very hard and durable. It takes a polish and was made to resemble ebony. The colour and grain of the Laburnum heart wood may vary according to soil and the age of the tree. It is darkest in the common Laburnum when grown on poor calcareous soils and lightest in Laburnum alpinum when grown in deep rich soil, in which case the colour is a sort of greenish black and was much in demand by cabinet makers and was employed in the production of musical instruments particularly flutes.

The tree does not branch very copiously as many of its small side buds give rise to short dwarf shoots marked with closely ranged ring scars of the bud scales.

The Roots---Digging near the roots of old Laburnum trees,it is probable to come across curious coral-like masses of tubercles which are attached to the roots,and which are often taken as being galls. These exostoses as they are termed,occur on the roots of almost every member of the order Leguminosa {Fabels}, but are particularly large on the roots of Laburnum, the clusters in this case being often a couple of inches across.

They are metamorphosed lateral rootlets and are produced by the attacks of certain lowly microscopic fungi or bacilli known as Rhizobium. These bacilli live in, and to some extent,upon, the tissues of the Laburnum root,causing them to grow out into swellings. It is not a mere parasite for the tree suffers no apparent harm,and indeed, is thought to profit from them.

The bacilli { as in the case with clovers} have the property of assimilating nitrogen from the air and it is thought the tree actually consumes or digests the bacilli in its root cells with the nitrogenous compounds they contain. They live together for mutual benefit and as far as man is concerned it is one of those trees that, instead of exhausting the soil in which they grow they actually enrich it, either by the decay of the roots or when ploughed in as a 'green manure'.

Flowers and foliage of Laburnum anagyroides

uploaded to Commons by Biopresto
uploaded to Commons by Biopresto | Source

Foliage of Laburnum

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The foliage

The leaves and flowers occur simultaneously in May, the former being grouped together in tufts. The leaf stalks are long and each of the three elliptical leaflets are borne on short stalks known in botanical parlance as petiolules.

The central leaflet has a slight joint or articulation at its base which indicates that this compound leaf,unlike that of the clover,belongs to the pinnate kind. two small persistent stipule's occur at the base of the 'leaf' stalk, and the young shoots and the under surface of the young leaves are alike covered with a silvery coating of flat silky hairs. The leaflets up to three and and a half inches long {nine cm}.

Laburnum flowers in bud

Uploaded to Commons  from Flickr by Magnus Manske
Uploaded to Commons from Flickr by Magnus Manske | Source

The flowers

Laburnum's produce a profusion of blossoms which appear in May in the case of L.anagyroi in the case of L.alpinum they continue into July. They hang in clusters up to just under a foot long in some cases. They are golden yellow and so many are produced that the foliage is almost hidden and they are certainly the glory of the tree.

It is noticeable that each flower bud,before it opens is inverted,as if intended to be an unpright flower spike.,so that as it opens,it has to twist through an angle of 180 degrees,much as do the flowers of most orchids. While the cluster hangs vertically downwards each flower stands horizontally,and its stamens and pistils are enclosed,within the keel-petals and so protected from the rain.

The 'honey' is secreted by the inner surface of the filaments near the base,and thus accumulates in the tube which they form around the ovary.In the related genus of Cytisus and a few allied groups,such as the gorse,all ten stamens are united, the tube of Laburnum flower is a complete one,so that the honey is effectually concealed at some depth within the flower and only tenacious insects with long tongues can procure it.

The anthers,moreover, ripen their pollen rather before the surface of the stigma shows by its stickiness that it is ready to receive the pollen grains. Just below the stigma is a circle of hairs which may hinder the pollen of the stamens from reaching it. This complex structure is a special adaption for cross-pollination by bees. Alighting upon the wing petals, the insects depress them by their weight ,and as the wing petals are moulded with a protuberance fitting into a corresponding hollow in the keel-petals, the latter are depressed at the same time.

This exposes the stamens and stigma until the bee's weight is removed when they will become covered once more by the keel-petals. In freshly expanded flowers only the short stamens are protruded and the and they dust the abdomen of the bee with pollen. However, when the flower is a day or two older the pistil and longer stamens emerge and deposit the pollen on the back of the bee. Thus, the pollen obtained at these two stages of the flowers development will reach the stigmas of other flowers differing in the length and progressive curvature of the styles.

To reach the honey the bee bores through a swelling just below the standard petal. Possibly at a later stage ,if not already cross-pollinated, the stigma may,by this curvature of the style be brought into contact with any remaining pollen of its own flower.

The flowers are fragrant.

The Pea-like flowers of Laburnum

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The fruits ,Legumes { seeds and seed pods}

The blossoms are followed by silky green pods which turn black as they ripen,then they burst elastically by contraction on drying of certain oblique groups of cells in each valve. Thus the black seeds are scattered to some little distance.

All the green parts of the tree are poisonous,containing an irritant emetic principal known as Cytisin, but this appears to exist in a particularly concentrated form in the seed. { The seeds are very toxic to children and also to dogs}.

In its germination the seed resembles the bean rather than the pea,since the cotyledons rise above the ground,become green and act as the two first foliage leaves,whereas those of the pea remain as mere store houses of nutrients within the seed.

These seed leaves are stalk-less,elliptical,and fleshy and in no way resemble the subsequently produced foliage,and, a slight want of symmetry in the seed renders the two halves of each cotyledon unequal.

Pods and seeds

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Laburnum alpinum

This species flowers from May until June sometimes into the first week of July. The clusters of vanilla scented,pea-like flowers have both male and female flowers { In botanical parlance hermaphrodite} and are pollinated by insects particularly bees.

The yellow flowers are succeeded by fruit pods or legumes are green at first becoming shiny black upon maturity. As with the former species all parts of the plant are poisonous,particularly the seeds.

This species prefers well drained light or sandy soil or medium loam,but will grow less vigorously in other types of soils such as heavy clay or nutritionally poor soils. It will tolerate light woodland or full sun. As with the former species {see roots above} it has a symbiotic relationship with bacteria which extracts nitrogen from the atmosphere.

Laburnum alpinum

Taken in Prague in the Czech republic
Taken in Prague in the Czech republic | Source

Laburnocytisus

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Laburnum and the Gardener

Of constant interest is Purple Laburnum to the gardener in particular,which may bear three types of blossoms. The tree in the image bu Simon Garbutt,{ above} was created by grafting a purple broom Cytisus purpureus on to the common Laburnum and has the flowers that are typical of both plants and some that are intermediate between the two.

It originated in Paris in 1828,when Mr. Jean Louis Adam inserted the bud of a purple broom into Laburnum stock. Apparently a 'grafted hybrid' was formed, the two species becoming intimately united in their growing tissue or cambium.

The most common ornamental tree is a hybrid between this species and laburnum alpinum. This combines the longer racemes of L.alpinum with the dressier flowers of L.anagyroides,it also has the benefit of low seed production.It has gained the Royal Horticultural Society's Award of Garden Merit.

Laburnum x watereri the Voss's laburnum {Vossil} is another popular garden species.It is a deciduous spreading. The leaves consisting of three leaflets,which are glossy and deep green, Pendent chains of large yellow flowers are borne in late spring and early summer. This species attains the height of thirty feet with an equally sized spread. It likes to be in full sun.

Laburnum x watereri

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6 comments

D.A.L. profile image

D.A.L. 2 years ago from Lancashire north west England Author

aviannovice,

Hi Deb although every part of them is poisonous they are the most beautiful trees .Thank you for visiting. Best wishes to you.


aviannovice profile image

aviannovice 2 years ago from Stillwater, OK

These are such beautiful trees. The yellow offsets the green perfectly. Great work.


D.A.L. profile image

D.A.L. 2 years ago from Lancashire north west England Author

Eiddwen,

Hi Eddy, Thank you your visits mean a lot. Hope you have a good week in your little corner of Wales. Best wishes to you.


Eiddwen profile image

Eiddwen 2 years ago from Wales

A wonderful hub my friend. I have always love the Laburnum tree or Golden Chain I didn't know too much about it (until now of course) Thank you once again for a wonderful lesson and as always such a well presented hub. Here's wishing you a wonderful day from across the border.

Eddy.


D.A.L. profile image

D.A.L. 2 years ago from Lancashire north west England Author

Hello Devika,

Glad you enjoyed it. Thank you for being the first to visit and for your kind comments they mean a lot. And for your votes which are appreciated..


DDE profile image

DDE 2 years ago from Dubrovnik, Croatia

Great photos! You surprised me with this hub! I enjoyed another learning lesson from you about trees. A perfect display of information and most educational. The yellow flowers are so beautiful.Voted up, interesting useful and beautiful.

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