of Dioscorea prehensilis . 417 
a single row of elements somewhat larger than the surrounding 
cells. Only one case was seen in which the appearance of 
Fig. 19. — 1-6 illustrating the origin of one of the central vessels of the large spine- 
bearing roots. 7-12 showing the cell- walls within the lumen. 
the first origin of a central vessel suggested an initial 
absorption of the lateral walls of contiguous cells (Text- 
Fig. 19, 1-6). Increase in size takes place, and sooner 
or later transverse walls, in varying stages of completeness, 
are seen stretching across the lumen. The majority of these 
