THE STRUCTURE OF THE ROOT 25 



Thus in the construction of all cells two ends have to be 

 kept in view : firstly, the rigidity of the organ, and secondly, the 

 facility of passage of the nutritive fluids. 



The structure of the wall, however, shows us other very 

 useful arrangements, and to these belong the valves which 

 have received the name of bordered pits, and which regulate 

 the amount of water which may pass from cell to cell. First 

 of all we have to grasp the fact that in the case of two adjoin- 

 ing cells or vessels the pit in one cell always corresponds in 

 position to a pit of the adjoining cell. This is not shown in 

 Fig. 7, because at the side of the large vessel there is no 

 second vessel, but only thin parenchymatous cells. The current 

 of water has, therefore, only to pass through the thin primary 

 membrane, which is common to the two cells and separates 

 the two pits one from the other. The communication of two 

 pitted vessels or cells resembles, therefore, a room which has 

 been divided into two compartments (cells) by a piece of 

 outstretched calico. But it has been found desirable to 

 strengthen the calico by covering it with a layer of mortar on 

 either side. So as to ensure communication between the two 

 compartments, however, glass rods and glass funnels have been 

 placed at right angles to the sheet, meeting each other, so that 

 the space they protect should not be covered by the lime. 

 When the thickening is completed, we may consider the rods 

 and funnels broken away or withdrawn, and the wall will then 

 be ready for its functions. Wherever a glass rod was pre- 

 viously fixed, a pit will be found, at the bottom of which the 

 sheet will be visible. Two such pits on either side of the sheet 

 will form a cylindrical canal ; but where two glass funnels 

 touched the sheet with their broad bases, the canals will 

 become narrower towards the room itself and the two canals 

 will therefore form a lens-shaped hollow, in the centre of 

 which will be stretched a piece of the calico sheet. 



If we were to introduce a hand into such a narrow canal, 

 and to push the sheet towards the other room, we should be 

 able, if the sheet were sufficiently extensible, to push our fist 

 so far that it would come in contact with the narrow neck of 

 the funnel-shaped opening on the other side. In that case 

 the fist would close the opening. 



