BOTANY 



PART I 



inner circles. The pit membrane is frequently thickened in the 

 centre forming the TORUS (Fig. 71 (7), and this, when the membrane 

 is deflected to one or other side, may close the entrance like a valve 



(Fig. 71 B, t). The wide mem- 

 brane of the bordered pits allows 

 readily of movements of water 

 from the one cell cavity to the 

 while the overhanging 



FIG. 69. A, Diagrams of tracheae in longi- 

 tudinal section. I., Wide trachea with 

 small elliptical bordered pits, and with 

 simple perforation of the end wall (q, q). 

 The further portion of the wall is cut away 

 in the upper portion of the figure. II., 

 Narrow trachea with scalariform pitting 

 of the wall and perforation of the trans- 

 verse wall, q. B, The transverse walls 

 of the two tracheae in surface view. 

 (After ROTHERT.) 



FIG. 70. A, Lower third of a 

 scalariform tracheide from the 

 rhizome of the Bracken Fern 

 (Pteris aquilinn) ; t, the trans- 

 versely-extended pits on the 

 lateral walls ; q, the scalari- 

 form pitted end wall, (x 95. 

 After DE BARY.) B, A 

 tracheide with circular bor- 

 dered pits, (x 100. After 

 STRASBURGER.) 



wall of the pit ensures that the rigidity of the wall is not unduly 

 diminished. 



As Fig. 71 shows, the pits are bordered on both sides of a wall 

 separating two water-conducting elements. When, however, a vessel 

 abuts on a living cell, the pit is only bordered on the side of 



