THE MATERIAL INCOME OF PLANTS 319 



passages are formed gradually among the parenchyma cells by partial 

 separation as they enlarge. At first all cells are coherent with their 

 neighbors, a necessity of the mode of division; but unequal growth 

 and turgor produce strains which split the common wall at the corners 

 and sometimes along whole faces (fig. 627). In submersed water plants 

 the aerating system attains its most marked development; huge canals 

 arise in the softer tissues of the stems and leaf-stalks (fig. 628), and in 



FlG. 627. Cross section of leaf of lily, somewhat diagrammatic: e, upper epidermis ; ', 

 lower epidermis, with stomata, s, in cross section; p, palisade; between p and e', spongy 

 tissue, with large intercellular spaces (*) below stoma (s) and vein (v). From PART I. 



other parts branched cells, the branches in contact only by their tips, 

 leaving large space for gases. These inner chambers in submersed 

 aquatics do not communicate with the atmosphere directly ; they con- 

 tain gases which have come out of solution in the adjacent cells and 

 constitute an internal atmosphere into which gases may diffuse or from 

 which gases may migrate into the living cells (of course in solution), 

 (See further, Part III, p. 551.) 



