38 



COMPENDIUM OF GENERAL BOTANY. 



facilitated by means of the pores. Secondarily it may occur that 

 cell-sap is conducted through the spiral vessels and other matured 

 mechanical cells during the entire life-period of the plant. In such 

 cases pores are a necessity for mechanical cells. Nevertheless it is 

 evident that porosity does not materially interfere with the proper 

 function of such cells. In typically mechanical cells pores are 

 therefore scarcely noticeable. As a rule they extend diagonally, 

 more rarely longitudinally. 



We shall now consider a new form of pore. Pores heretofore 

 considered (Fig. 16) are called simple, ordinary or uiibordered pits 

 or pores. This new form of pore is called a bordered pore or pit. 





 

 



FIG. 17. 



a shows a bordered pit in cross-section, which also explains the appearance in surface view 

 as shown at b. 



The two pore canals of bordered pits meet funnel-like, that is, the 

 two large thin membranous surfaces come in contact. This mem- 

 brane is therefore much larger than the diameter of the pore-canal 

 (see Fig. 17, a). 



Physiological anatomy still has here an opportunity to solve an 

 interesting problem. For the present we must be satisfied with 

 the suggestions of the greatest tissue physiologist, SCHWENDENEK, 

 in regard to the structure of these bordered pits. Schwendener sup- 

 poses the following arrangement. The presence of a large thin 

 membranous area to facilitate the exchange of fluids while the 

 cell retains the greatest possible firmness. This seems to be a 

 very rational explanation. However, as already stated, we have no 

 exact knowledge of the function of these structures. Another 

 hypothesis (Russow) supposes an arrangement for ventilation which 

 is based upon the presence of the u torus," that is, a thickened 

 central portion of the closing membrane (see Fig. 17, a). This 

 torus may be forced to one side or the other of the pore- 

 canal entrance by excessive pressure. Schwendener succeeded in 



