67 



sclereides. Simple pits may, on the other hand, expand on 

 approaching the primary cell wall. 



Pits widened towards the membrane are found in the external 

 cell walls of many tendrils ( 4S ). These pits, which are filled with cyto- 

 plasm, probably receive 

 the stimulus, and may 

 be termed tactile pits 

 (Figs. 68, 69). The 

 structures known as 



BORDERED PITS (Fig. 70) 



are a special type of 

 simple pits widened to- 

 wards the pit membrane. 

 The pit may be present 

 on one or both sides of 

 the wall; the former is 

 the case when the water- 

 conducting element 

 abuts on a cell with 

 protoplasmic contents, 

 the latter when the 

 pitted wall separates two water-conducting elements. In bordered 

 pits the closing membrane is thickened at the centre to form a TORUS 

 (Fig. 70, B). By the curving of the closing membrane to one side or 

 the other the torus may so act as to close the pit canal (Fig, 70 P>, f). 

 The bordered pits apparently act as valves. Seen from the surface a 



bordered pit appears as two con- 

 centric rings (Fig. 70 A). The 

 smaller, inner ring represents the 

 narrow opening of the pit into the 

 cell cavity ; the larger, outer ring 

 indicates the widest portion of the 

 PIT CHAMBER when it abuts on the 

 primary cell wall. 



The pit membrane of specially 



Pto. 75.- Epidermal cell from the margin of a ., . , ,, ., , . 



r-uiicai leaf of a,,,,,,,,,,,,!,, ,..,*;,//./,. The wide P8 between cells with thin 



Km. 74. Purl of transverse section of a stem of Inipatien.* 

 pai-viflora. e. Epidermis ; c, colleiicliynia ; )>, tliin-walk'<l 

 parenchymatuus ct-Ils ; i, intercellular sjwce. (x 300.) 



lens-shaped thickening 

 in this plant silicitied. 

 X 515.) 



of the outer wall is 



(After HABERLANDT, 



walls often shows thicker bands 



wh j ch g j ye i( . ft l att j ce _ wor k appear- 



ance. A similar type of thickening 

 is found in the membranes of the sieve-pits which, on account of 

 their being perforated like a sieve, are termed sieve-plates (Fig. 72). 



In cases where the greater part of the cell wall remains un- 

 thickened, its character is determined by its thickened rather than 

 by its unthickened portions ; it is in this sense that the terms 

 annular, spiral, and reticulate are used (Fig. 73). Just as in the case 

 of cells with bordered pits, annular, spiral, and reticulate cell walls 



