[ i83 ] 



CHAPTER XIV 



THE AERATING SYSTEM IN THE TISSUES 

 OF HYDROPHYTES 



THE existence of a highly-developed system of inter- 

 cellular spaces, is one of the most marked anatomical 

 characters of water plants. It is generally assumed that this 

 lacunar system serves for the storage of the oxygen evolved in 

 assimilation, and its conveyance to the parts of the body that 

 stand in especial need of it, more particularly the roots and 

 rhizomes buried in the asphyxiating mud. The mesophyll of 

 the lamina, the ground tissue of the petiole, and the cortex 

 of the stem and root, are the regions in which the air spaces 

 reach their greatest development. 



In the stem, the cortex, which is generally broad in propor- 

 tion to the stele, is penetrated by lacunae, which may be so 

 numerous as to render the whole organ extremely fragile in 

 texture. Two features in the arrangement of the cortical cells, 

 however, seem in some degree to obviate the dangers of this 

 fragility. The air spaces are, in the main, confined to the middle 

 cortex, while the outer cortex in many cases consists of elements 

 which are more closely placed and thus form a firmer peripheral 

 shell^; the septa, again, are radially arranged and thus are able 

 to withstand pressures acting at right angles to the axis, which 

 would otherwise be liable to crush the stem^. Support is also 

 obtained by diaphragms^, occurring chiefly at the nodes, which 

 divide the air spaces into sections; these diaphragms are not 

 air-tight, but are more or less water-tight, so that they form a 

 safeguard against the flooding of the entire aerating system in 

 the case of accidental injury. Fig. 1 1 8, p. i 84, represents part 

 of a transverse section of a stem oi Potamogeton natans^ in which 



1 Haberlandt, G. (1914). 2 Schenck, H. (1886). 



3 Duval-Jouve,J. (1872}, Blanc, M.le(i9i2)andSnow,L.M.(i9i4). 



