THE MATERIAL INCOME OF PLANTS 
3 J 9 
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 ; e', 
lower epidermis, with stomata, s, in cross section; p, palisade; between p and e', spongy 
tissue, with large intercellular spaces (i) below stoma (5) and vein (y). 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.) 
