234 



EXPERIMENTS WITH PLANTS 



through these pits (which are openings in the wall 

 closed by very delicate membranes) that the starch 



passes in the 

 form of dis- 

 solved grape- 

 sugar from 

 cell to cell. 

 These pits 

 occur on all 

 the cell-walls 



186. Bordered pit, cut in lialf (diagrammatic). of thc WOOd 



and are shown in the cross -section as dots in the cell- 

 wall (for details see Fig. 136, which is a diagrammatic 

 representation of one of these bordered pits cut through 

 the center; a similar section of a simple pit is shown 

 in Fig. 137) . These figures make it clear that the pits 

 are nothing more than holes in the wall across which 

 are stretched delicate membranes which allow water to 

 pass through but prevent the passage of air- bubbles, 

 starch grains, etc. On 

 looking at a bordered pit, 

 the narrowed entrance to 

 the hole appears as a 

 smaller circle within the 

 larger one which corres- 

 ponds to the diameter of 

 the hole at its widest 

 part. It is possible that 





137. Simple pit, cut iu half (diagrammatic] 



