28 MINNESOTA BOTANICAL STUDIES. 



which wind about and intertwine somewhat with each other. 

 These filaments anastomose frequently and are often dichoto- 

 mously divided (PL IX., -Figs. J, 4, 13). The cells of the 

 pith in the central part of the stipe or blade are of nearly 

 uniform diameter throughout their length and are regular in 

 shape, except at anastomosing plexi and near the cortex where 

 they are subject to distortion in shape and to displacement. 



The pith cells contain a few chromatophores collected into a 

 little pellet near the middle of the cell. The protoplasmic con- 

 tents show up well and numerous refracting grains of reserve 

 material are seen. 



The gelatinous intercellular matrix swells up greatly when 

 the plant is placed in fresh water. This causes the lamina to 

 burst open, beginning at the more tender tip of the lobes in the 

 young laminse. The stipe having a firmer cortex and also pro- 

 portionately less intercellular gelatine, does not burst, though it 

 swells some. 



Anastomosis and Pits. — Anastomosis is seen best in the pith 

 cells (PL IX., Figs. 2-4, 12-14). Sometimes two filaments 

 will simply be bent toward each other, touching with their con- 

 vexities (Fig. 2). At this point there is no intercellular jelly 

 separating the cells. A thin communicating plate is between 

 the cells in contact, and the protoplasm in these cells sends out 

 branches that meet at the plate. At other times the anastomos- 

 ing cells send out lateral protuberances, which passing through 

 the jelly, meet and form a pit at the point of contact (Figs. 4, 

 13, 14). Probably these protuberances were not formed before 

 the pit was formed, but are the result of the growth and modi- 

 fication of shape of the cells which took place after the pit 

 already was made. Judging from the conditions in the cortical 

 layers, these pits are simply the original fission walls left un- 

 thickened at these spots (Fig. 12), while at other points the 

 contiguous cells were forced apart by the development of the 

 gelatinous middle lamella between the two cells, however, leav- 

 ing the cells in contact at the pits. The pits at lateral anasto- 

 mosing points are smaller than at the ends of the vertical cells. 

 The pits do not stain as deeply as the rest of the cell wall, but 

 this might be due to their greater thinness. They are sharply 

 marked off in the walls of the pith and the inner cortex cells. 

 They are round or oval plates which in optical section are of 

 uniform thickness, not lenticular. They show less definition of 



