Natural History 611 



The Story of the Whin 



Green plants, too, may be constantly associated with partners 

 belonging to lower groups. If we dig up a young whin or broom 

 and examine the roots we find that they bear a profusion of warts, 

 ranging from the size of a pin-head on the smaller rootlets to 

 lumps the size of a pea on the larger. These are abnormal swell- 

 ings of the root tissue caused by the presence of a particular race 

 of bacteria. The bacteria enter the root hairs from the soil, and, 

 multiplying, travel inwards till they reach the rind-cells of the 

 root. There they increase and pass from cell to cell as the root 

 tissues enlarge in response to the irritation of the invaders. A 

 mature wart or nodule cut across and examined with the micro- 

 scope shows the cells packed with myriads of bacteria, and it has 

 been proved that these are able to fix the nitrogen of the atmo- 

 sphere always present in solution in the water which enters the 

 plant and to convert it into organic compounds. Part of the 

 supply is at the service of the host, and in return the bacteria are 

 supplied with carbohydrate food. This extraordinary association 

 is another example of symbiosis a mutually beneficial internal 

 partnership. 



These bacterial nodules are found on the roots of all members 

 of the great family of the Leguminosa?, the pod plants, to which 

 belong broom, whin, lupin, pea, bean, vetch, rest-harrow, and 

 many others familiar as wild flowers or important as food plants. 

 The members of the Leguminous family number some eleven 

 thousand and are to be found over the whole globe. But their 

 success may be appreciated by anyone who has travelled only as 

 far as the nearest common, or even a little way along the road 

 leading to it. For the heath and the common are clad with whin, 

 and the road is flanked with broom, shrubs the luxuriance of which 

 in stature, blossom, and seed is at strange variance with the un- 

 promising conditions of soil on which they flourish. It is the 

 possession of a private store of nitrogen compounds, supplied by 

 the bacteria, which makes this success possible. 



