i8 4 AERATING SYSTEM [CH. 



the cortical lamellae are connected by a diaphragm (Z>) with 

 small intercellular spaces (m) at the angles of the cells. Fig. 119, 



FIG. 118. Potamogeton natans, L. Part of T.S. of stem with diaphragm (D) 

 penetrated by intercellular spaces (m). f.l.b. = vascular bundle. [Blanc M. le 



(1912).] 



FIG. 119. Hippuris vulgaris, L. Three stages in the development of the nodal 

 diaphragms of the stem, seen in T.S. (all x 318). A, young stem, intercellular 

 spaces small and walls scarcely thickened. C, old stem, 7 mm. in diameter; inter- 

 cellular spaces so much enlarged that the cells are stellate, walls much thickened. 

 B, same stem as C, but from a region 3-5 mm. across, which shows intermediate 



characters. [A. A.] 



A y 5, C, shows the development of the nodal diaphragm-tissue 

 in the case of Hippuris. 



The air spaces may be either formed by the separation of 

 cells (schizogenous) or by their destruction (lysigenous). When 



