THE INTERCELLULAR SPACES 



131 



some vapor of water passes out through the lenticels, but that 

 they are needed for this seems improbable. 



The Intercellular Spaces. — If there were no intercellular 

 spaces threading the plant body 

 in all directions the cells lying 

 remote from the surface would 

 have access to the gases of the 

 atmosphere only as they came 

 by slow diflFusion in solution in 

 the cell sap, and this would be 

 inadequate for all cells that are 

 actively living and working, such 

 as those cells that construct and 

 store, digest and otherwise trans- 

 form food, and all cells that are 

 passing through cell-division, 

 growth and differentiation. And 

 so air spaces are formed between 

 cells throughout the whole plant 

 body from tips of roots to tips of 

 branches and from stomata or 

 lenticels to the pith. The plant 

 is, in fact, riddled with these fine 

 spaces to such an extent that 

 probably the cells farthest from 

 them are distant not more than 

 a few tenths of a millimeter. 



The formation of the inter- 

 cellular spaces begins in the 

 meristematic tissues before the 

 differentiation of the permanent 

 tissues has begun; and this 

 should be expected since the 



dividing and growing cells of the meristems respire rapidly and 

 make relatively large demands for oxygen and the elimination of 

 carbon dioxide. 



Fig. 67. — Diagram suggestive of the 

 distribution of intercellular spaces 

 throughout a plant. The heavy hori- 

 zontal lines in stem and root indicate 

 intercellular spaces in medullary rays. 

 The arrows indicate the entrance of gases 

 through the stomata of the leaves and 

 stomata and lenticels of the stem, and 

 anyw'here over the roots, which are less 

 water-proofed than the leaves and stems. 

 Lenticels have been found where the 

 lateral roots grow out from the main root. 



