AERENCHYMA. DIAPHRAGMS 443 



air-cavities, which collectively form a continuous cylindrical air-jacket 

 (Fig. 166). The successive jackets corresponding to the concentric 

 layers of aerenchyma, communicate with one another radially, hy means 

 of intercellular spaces which intervene between the tangentially 

 expanded bodies of the aerenchyma cells. As seen in radial 

 longitudinal section, therefore, each cell appears I shaped, the 

 horizontal stroke representing the arm which traverses the air-jacket 

 surrounding the particular layer to which the cell belongs, while the 

 vertical stroke corresponds to the expanded body of the cell. 

 Tangential connection between neighbouring cells of each layer is 

 effected by those elements which do not become produced into radial 

 arms. In these roots of Jussiaea, both the epidermis and the 

 hypodermal layer (the cells of which do not undergo any radial 

 extension) disappear at an early stage ; the ventilating system thus 

 soon opens directly into the surrounding water. As in other cases, 

 the cells of the aerenchyma are not themselves full of air, but are, on 

 the contrary, provided with a thin peripheral layer of protoplasm, 

 which contains a nucleus and minute leucoplasts, and encloses a sap- 

 cavity filled with a watery liquid. 



In Jussiaea, and in certain other Onagraceae and Lythraceae, 

 a very similar aerenchyma is also developed on the so-called " floating " 

 roots, and on the submerged portions of the stem. In these latter 

 cases, only a small portion of the aerenchyma consists of primary 

 cortex, the rest being produced by a secondary meristem, which is 

 homologous with the phellogen. Kosanoff long ago showed that the 

 aerenchyma of the submerged stems of Desmanthus nutans is a secondary 

 tissue. 



A different type of aerenchyma is exemplified by the felted stellate 

 tissue which fills the air-passages in certain Cyperaceae, notably in 

 species of Scirpits ; the tissue in question also performs a mechanical 

 function, by helping to prevent the collapse of the air-passages, and to 

 preserve the cross-sectional outline of the entire organ (cf. p. 181). 



5. Diaphragms and internal hairs. 



Air-passages are often interrupted at intervals by diaphragms, 

 composed of one or more layers of aerenchyma. Besides acting as 

 stiffening plates (cf. p. 182), these diaphragms play an important part 

 in connection with ventilation. They are always more or less lacunar, 

 in structure, and hence do not in the slightest degree interfere with 

 the movement of gases along the air-passages ; in the haulms of 

 Papyrus, they are actually responsible for effecting communication 

 between adjacent ventilating chambers, since each diaphragm extends 

 across several air-passages. Whereas, however, such diaphragms are 



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