342 FIXATION OF FREE NITROGEN BY BACTERIA. 



was slow. Both classes were under the influence of Boussingault, 

 whose experiments (1837 to 1858) in the cultivation of legumi- 

 nous and other plants under bell-glasses in soils destitute of 

 nitrogen gave results confirmed by the check experiments of 

 Lawes and Gilbert which seemed to deny the absorption of free 

 atmospheric nitrogen by plants. 



Shortly afterwards, however, attention was directed to the 

 bodies now termed leguminous nodules. These are lateral ap- 

 pendages or swellings on the roots (as shown in Fig. 59), and 

 occur both on the younger and older portions of same, the former 

 being the most thickly infested. In one and the same plant all 

 the intermediate stages of this formation, ranging from such as 

 are just barely perceptible to those as large as a pea or hazel-nut, 

 can be found. Their form (Fig. 60) differs in different species, 

 observations on which point have been published by A. TSCHIRCH 

 (I.) and by LAWES and GILBERT (I.). 



.The earliest description of the leguminous nodules was given 

 by MALPJGHI (I.) in his book, published as far back as 1687, and 

 this observer referred to them as galls, i.e. diseased excrescences, 

 an opinion also shared by P. DE CANDOLLE (I.) in 1825. TRE- 

 VIRANUS (I.), in 1853, was the first to regard these nodules as 

 normal growths, and thirteen years later they were studied by 

 WORONIN (I.), who made the (subsequently important) observa- 

 tion, that the formation contains entirely-closed cells filled with 

 living bacteria. In the seventies ERIKSSON (I.) and CORNU (I.) 

 recognised these appendages as metamorphosed lateral roots of 

 perfectly unique structure. 



When regarded in section (Fig. 6i)> a nodule of this kind is 

 seen at the first glance to consist of two different portions a 

 white or colourless external zone and an interior layer, pale red in 

 the young nodule, but afterwards greenish-grey, the line of de- 

 marcation between them being somewhat sharply defined, and the 

 outline indented like that of a blackberry. It is in the cells of 

 this inner layer that the bacteria now under consideration, and 

 more fully described below, are sheltered; and the layer itself 

 is known as the bacteroidal tissue. On account of these enclo- 

 sures the said root-nodules received from A. B. FRANK (III.) in 

 1879 tne name of mycodomatia (i.e. fungus chambers), an expres- 

 sion that has, however, been abandoned. 



193. Formation and Functions of the Nodules. 



The first to take under consideration the physiological import- 

 ance of these nodules was LACHMANN (I.), who in 1858 defined 

 them as stores of albumin. One-and-twenty years later FRANK (III.) 

 showed that the formation of nodules does not occur when the 

 plants are grown in sterilised soil, thus proving that the co-operation 

 of microbes existing in the soil is a necessary factor. This result, 



