AERENCHYMA 



175 



chyma is a very lacunar secondary tissue, formed in place of 

 cork by the phellogen, which cuts off cells only on the outside. 

 These remain thin-walled and living and, as they enlarge, par- 

 tially separate, and so produce a system of wide air-spaces (often 

 concentric in their arrangement) to which the spongy character 

 is due. The function of this tissue is to^ supply air to the sub- 

 merged parts, and it is particularly well developed in plants 

 growing in water-logged 

 soil. 



The numerous large 

 air-canals of aquatics are 

 often segmented by plate- 

 like septa or diaphragms 

 composed of many small 

 cells separated by minute 

 intercellular perforations. 

 These latter are too small 

 to admit of the passage 

 of water, and thus pre- 

 vent the injection of the 

 air-canals, when fragments 

 of water-plants become 

 detached, as normally 

 occurs in vegetative re- 

 production ; owing to the 

 perforations the flow of 

 air is not obstructed. 



Aquatics display a re- 

 markable plasticity of 

 structure, particularly in 



relation to their anatomy, as a result of which many can grow 

 either completely submerged or on the mud near the edge 

 of the water. A comparison of such land- and water-forms, 

 belonging to the same species, clearly shows the adapt at ional 

 significance of most of the characters of aquatics. Thus, in a 

 cross-section of the stem of the land-form of the Water Starwort 

 (Callitriche, Fig. 93, a), the cortex consists of closely packed 

 rounded cells with small intercellular spaces between them, 

 whilst that of the water-form (Fig. 93, d) is mainly occupied by 



FIG. 92. Photomicrograph of a small 

 portion of the aerenchyma (a) of the 

 Marsh Samphire (Salicornia.) Co., 

 cortex ; Xy., xylem. (Photo E. J. S.) 



