PHYSIOLOGY OF NUTRITION. 



find that its roots bear frequently a very considerable number 

 of tubercles, sometimes of large size, which at this time are 

 generally rose- red in colour. If we examine our pea plants grown 

 in sand, we shall find that only the plants provided with un- 

 sterilised soil-extract have tubercles on their roots. 



We now prepare median transverse sections of well-developed 

 rose-red tubercles from pea roots, and examine first under lower 

 and then under higher magnification. The middle of the section 

 is composed of a Very large-celled tissue, provided with inter- 

 cellular spaces, whose cells are rich in plasma. In addition we 

 perceive traversing this tissue, which is designated bacteroid 

 tissue, characteristic branched highly glistening threads. To- 

 wards the outside the bacteroid tissue is bounded by a layer of 

 parenchyma, which, on account of the starch contained in its 

 elements, may be characterised as the starch layer. In this 

 parenchyma run fibrovascular strands, which, as recent researches 

 have taught, are in connection with the fibrovascular bundles of 

 the roots bearing the tubercles. It is specially to be noted that the 

 bast portions of the fibrovascular strands are directed inwards, i.e. 

 towards the bacteroid tissue, while the wood is directed outwards. 

 A superficial cortex is developed on the tubercles, which is com- 

 posed of several layers of cuticularised cells. 



If it is desired to study the developmental history of these 

 tubercles in the Papilionacese, and the phenomena connected with 

 their ultimate emptying, the student is referred for further infor- 

 mation to Prazmowski's work (Versuchsstationen, Bd. 37, p. 209), 

 since the space at our disposal does not permit us to enter here on 

 this very complicated subject. It has been found that the highly 

 refringent filamentous structures already mentioned as traversing 

 the bacteroid tissue are tubes, which contain large numbers of 

 bacteria, Bacterium Radicicola. This Schizomycete occurs in the 

 soil, penetrates the root-hairs of Papilionaceous plants, here forms 

 the tubes, which now grow into the cortex of the roots and induce 

 the formation of tubercles. The tubes swell, and the bacteria 

 present in them, continually and copiously multiplying, pass over 

 into the bacteroid tissue of the developing tubercles, but after a 

 time they lose their power of dividing, are metamorphosed, and 

 form the so-called bacteroids, dichotomously branched corpuscles, 

 which occur often in immense numbers in the cells of the bac- 

 teroid tissue. 



Bacterium Radicicola, which stands in a symbiotic relationship 



