22 FAMILIAR TREES 



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 Rhizo'hium. These bacilli live in, 

 and to some extent upon, the tissues of the Laburnum 

 root, causing them to grow out into these swellings ; 

 but this is no mere case of parasitism, for the tree is 

 none the worse, but probably much better, for the 

 presence of the fungi. The bacilli have the property 

 of assimilating the free nitrogen of the atmosphere, 

 and the Laburnum apparently actually consumes or 

 digests the bacilli in its root-cells with the nitrogenous 

 compounds they contain. The association of the two 

 plants is, therefore, a case of symbiosis, or living to- 

 gether for mutual benefit ; and the important result, so 

 far as man is concerned, is that in this way legumin- 

 ous plants, instead of exhausting the soil in which they 

 grow, actually enrich it, either by the decay of their 

 roots or when ploughed in as " green manure." 



The hanging clusters of golden blossoms, some- 

 times nearly a foot in length, and so thickly set upon 

 the tree that its foliage is well-nigh hidden, are the 

 peculiar glory of the Laburnum. It is noticeable that 

 each . flower-bud before it opens is inverted, as if 

 intended for an upright flower-spike; so that, as it 

 opens, it has to twist through an angle of 180°, much 

 as do the flowers of most orchids. While the cluster 

 hangs vertically downward, each blossom stands 

 horizontally, and its stamens and pistils are enclosed 

 within the keel-petals and so protected from rain. 

 The honey is secreted by the inner surface of the 

 filaments near their base, and thus accumulates in the 



