PISUM SATIVUM - 347 



of a thread of bacterial zoogloea from which irregular masses of zoogloea 

 grow out within the cells. The thread itself becomes encased in a 

 sheath which is chemically similar to the walls of the host cortical 

 cells and is probably a secretion of the plant. Release of bacteria 

 from the ensheathed infection thread into the host cytoplasm takes 

 place by the formation of blister-like swellings of the sheath which 

 eventually bursts." 



The tubercle bacilli multiply rapidly; the thread-like infection 

 strand extends through the length of the hair to its basal portion; 

 and, from that point, continues the invasion through the under- 

 lying cortical parenchyma, the endodermis, and finally into the 

 pericycle. As a result of the penetration of this strand, the cells 

 of the pericycle proliferate and form a conical mass of meristematic 

 tissue which in its initial stages resembles the meristem of the 

 primordium of a lateral root. Because of this similarity, the 

 nodule has been regarded as the morphological equivalent of the 

 lateral root, but the resemblance is purely a superficial one. 



The dome-like mass of pericyclic tissue pushes out into the cor- 

 tex, producing a nodular swelling of the root axis at that point. 

 At first, the surrounding epidermal and cortical cells keep pace 

 with the growth of nodular tissue; but later they may become dis- 

 integrated as a result of mechanical stretching and abrasion. The 

 central parenchymatous and vascular tissue of the nodule is sur- 

 rounded by a well-defined endodermal layer, except at the tip, and 

 apical growth of the nodule by means of meristematic activity may 

 proceed for some time. The bacterial tissue occupies the central 

 portion of the nodule, and between this zone and the endodermis are 

 several layers of parenchyma in which two or three vascular strands 

 are differentiated. These abut the protoxylem points of the stele 

 of the root and extend the full length of the nodule to the meri- 

 stematic zone. 



The vascularity of the nodule in Pisum is similar in all essential 

 details to that described for Vicia Faba by Brenchley and Thornton 

 (i), who state that under conditions of normal nutrition, 



"The strands run the full length of the nodule, and merge into the 

 meristematic tissue, the smaller protoplasmic cells being distinguishable 

 for a rather greater distance than are the lignified elements. The 

 strands follow a sinuous course and often branch, and in cross section 

 as many as ten may appear, some of which in reality represent the 

 same strand cut through more than once owing to the curvature. No 

 evidence of anastomosis has been obtained, even in the large nodules 



