SYSTEMATIC POSITION OF MITBASTEMON. 205 



elongated wedge with a long thread at the apex. The thread which lies 

 deep within the xylem, after having finished its physiological function, is 

 found dead and buried deep in the xylem. The vertical threads are com- 

 posed only of rounded parenchymatous cells which are rounded at the basal 

 portions but by and by become elongate towards the apex, especially in the 

 apical portions which are composed of a single row of cells. 



The horizontal threads become more and more slender as they recede 

 from the flower-region, and show no more differentiation of peripheral paren- 

 chymatous and central tracheidal tissue, and in much remoter regions, say 20 

 or 30 cm. from the flower-regions and in the part of the root where it attains 

 nearly 2 mm. in diameter (in main or adventive roots), they remain as 

 threads composed each of a single row of cells in or just above the cambium- 

 layer. This shows how widely they are spread over the host root. They 

 are by no means straight, but always anastomosing and running in a zig- 

 zag manner. In longitudinal sections of the host-root, they appear, not as 

 straight lines, but as interrupted or dotted lines. The vertical threads are 

 usually larger in parts near the floral polster, but become shorter in remoter 

 regions. As the horizontal threads reticulate themselves, so do the vertical 

 threads in the region of the xylem, mostly at their apical portions towards 

 the center of the pith. There the vertical threads push out formless branches 

 towards the apex of the host-root, along its long axis, parallel to the 

 horizontal threads in the bast. Here we have another kind of horizontal 

 threads which connect the vertical threads in the xylem, making an irregular 

 network. This network is composed of irregular threads composed of single 

 rows of rounded cells which are very much larger than those in other parts 

 of the intramatrical tissue, and are of very irregular amoeboid shape. The 

 vertical threads which run usually side by side with the medullary rays are 

 easily distinguished from their companions by their having elongated rounded 

 cells full of plasmas, with much larger unclei, and smooth and much thinner 

 walls. The cells of the medullary rays, in contrast to those of the vertical 

 threads, are usually rectangular, and have pitted and thicker walls. The 

 intramatrical tissue is generally distinguished from the host tissue by its 

 taking a much deeper stain with any staining matter, for example with 



