In 1825 an experienced gardener in France, Jean Louis Adam, performed a minor miracle.  No doubt experimenting in his nursery, he tried to graft a broom onto a laburnum.  This could be a success since the two are botanically from the same family, commonly known as the Pea family.  

The results may not have been what he expected.  The graft on one plant appeared to have died, but it had actually taken on the outer part only, ie the cambium and phloem.

The cambium layer is a collection of young cells just inside the stem which continually split to form more new cells, some of which increase the size of the centre stem - the xylem.  The centre ones remain as cambium and the outer ones form and add to the soft layer just under the bark.  This part of the stem is the main area for carrying food material (sugars) manufactured in the leaves to the roots and other parts of the plant.  It is known as the phloem.

The graft in this case eventually produced a shoot which, instead of being completely broom, as planned, consisted of the outer layer, the phloem of the broom, and part of the cambium.  The inner part consisted of part of the cambium layer and xylem - being laburnum.  To simplify, it is an inner structure of laburnum covered with a 'glove' of broom.

An interesting feature of the adult tree is that the main growth habit is like a laburnum.  It produces shoots similar to the broom as well as two other types - laburnum and an intermediate type.  It produces three distinct flowers.  The yellow flowers of the laburnum (Laburnum anagyroides), purple flowers of the  broom (Cytisus purpureus) and flowers which are laburnum-type yellow and pinkish-purple in colour.

The smallish tree eventually grows to the height of 8m/25ft with a width of 6m/20ft, and is deciduous.  It will grow on almost any soils which are not waterlogged.  It was originally known as Laburnum adamii, but now officially its name is Laburnocytisus adamii.  It poses a slight problem with pruning, and I remember in the past suggesting to one of our most experienced gardeners that he was in fact pruning it wrongly.

Usually, if pruning is done to ornamental trees, it consist of cutting out weak shoots to give more light and air to the plant.  In this case removing the weak-looking shoots removes the broom component.  If you are unfamiliar with pruning, leave well alone. 

Not spectacular, it is grown mainly as a curio, and is reasonably easy to obtain.  It is described as a graft hybrid and chimera.  There is another similar: Crataego-mespilus which is a graft hybrid between hawthorn and medlar.