74 



BOTANY 



PART T 



other points, the original cell wall remains thin. In this way canals 

 are formed which penetrate the thickening layers. These pores or 

 PITS may be either circular (Figs. 72 w, 11 m), elliptical, or elongated. 



The pits in adjoining cells coin- 

 cide, and would form one con- 

 tinuous canal, were it not that 

 the unthickened primary cell 

 wall persists as a pit membrane 

 betAveen the two pits. The open- 

 ings of narrow elliptical jjits into 

 adjoining cells usually appear to 

 cross one another obliquely. As 

 a result of the continued thicken- 

 ing of the cell wall, the canals of 

 several pits often unite, and so 

 BRANCHED PITS are formed. Such 

 branched pits have usually very 



Fig. 80.— Portion of a loiigitndinal section through UarrOW Cauals, and OCCUr for the 

 three spiral vessels and a row of parenchyma niOSt part Only in extremely thick 

 cells of the Gourd (Cucnrhita Peno). (After , , i n n 'j. 



w. KoTHERT. X 560.) and hard cell walls, as, for in- 



stance, in those of the so-called 

 sclerotic cells or 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 (^^). These pits, which are filled with 

 cytoplasm, probably re- 

 ceive the stimulus, and 

 may be termed tactile 

 pits (Figs. 74, 75). The 

 structures known as 



BORDERED PITS (Fig. 76) 



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 proto- 

 plasmic contents, the 

 latter when the pitted 

 wall separates two water-conducting elements. In boi'dered pits the 

 closing membrane is thickened at the centre to form a torus (Fig. 76, />). 

 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. 76, B, I). The bordered 



Fig. 81.— Part of transverse section of a stem of Impafirvs 

 parviflora. e, Epidernii.s ; r, colleiichyma ; ji, thin-walled 

 ])arenchymatous cells ; i, intercellular space, (x 300.) 



