THE CELL. 37 



sow, STKASBURGER, and others). Upon these discoveries future 

 investigators may base important conclusions of a widely different 

 nature; as, for example, the transmission of irritability. This will 

 not, however, be further mentioned at this point ; we will main- 

 tain that pores facilitate the interchange of fluids from cell to cell, 

 as well as between cell and vessel (tracheid). 



These pores or thin cell-wall areas are often of circular or oval 

 form, again linear or simply fissure-like. The accompanying figure 

 represents various forms of ordinary pores. 



a, shows fissure-like pores ; &, right-hand side, shows rounded 

 and oval pores, all in surface view; Z>, left side, shows the corre- 

 sponding pores in profile, that is, in ver- 

 tical section through the cell-wall. Here 

 the pore is shown to be a canal. It may 

 be stated that as the rule pores of neigh- 

 boring cells meet each other ; to this there 

 are exceptions. 1 It is also the rule that a 

 these canals pass through the cell-wall at 

 right angles. Upon the number of pores in a given cell-wall area 

 the prevailing direction of the interchange of food materials may 

 be based. Without further elucidation it is evident that thin- 

 walled cells ( for example, most assimilating cells) do not require 

 pores, though there is extensive interchange of food-substances 

 between them and other cells. 



The rounded or oval pores are typical in those moderately thick- 

 ened elements which function chiefly in nutrition; such as the storing 

 and conducting parenchyma cells of pith and cortex, the storing and 

 conducting cells of medullary rays and wood-parenchyma. These will 

 be further discussed later. The above-named \\\\ewc fissure-like pores 

 are characteristic of mechanical cells. It is evident that cells which 

 are destined to withstand pressure or tension may still perform this 

 function though devoid of life, since the dead cell-wall constitutes 

 the most important part. The pores in mature mechanical cells 

 therefore appear to be harmful, since they form interruptions in the 

 continuity of the cell- wall substance. But so long as these elements, 

 which later subserve a purely mechanical function, grow, they must 

 be nourished, and indeed richly. The necessary supply of cell-sap is 



1 Such cases require further study. In advance it may be stated that their 

 explanation will probably throw light upon new adaptations. 



