MORPHOLOGY 



77 



the so-called sclerotic cells or sclereides. Simple pits may, on the other 

 hand, expand on approaching the primary cell wall. 



The structures known as bordered pits (Fig. 83) are but a special 

 form of such expanded simple pits. In bordered pits the closing 

 membrane is thickened at the centre to form a torus (Fig. 83, C). 



..--••■ 



Wi\ 





■-*»:■*!?■ 



Fig. S5.--Part of two 

 sieve- tubes • of the 

 Pine, Pinus sylvestHs, 

 showing sieve - pits. 

 (X 540.) 



Fig. 86. — A, Part of an annular tra- 

 cheid ; B, part of a spiral tracheid ; 

 C, longitudinal section through part 

 of a reticulate vessel, showing perfor- 

 ated partition wall, s. ( x 240.) 



By the curving to one side or the other of the closing membrane, the 

 torus may so act as to close the pit canal (Fig. 83, B). Bordered pits 



are only formed in cells 

 which are soon to lose 

 their living contents and 

 thus serve merely as 

 channels for conducting 

 water. The bordered pits 

 apparently act as valves. 

 Seen from the surface a 

 bordered pit appears as 

 two concentric rings (Fig. 

 83, A). The smaller, 

 inner ring represents the 

 narrow opening of the 

 pit into the cell cavity ; 

 the larger, outer ring 

 indicates the junction 

 of the wall of the PIT CHAMBER with the primary cell wall. 



Very large pits between adjoining living cells have often thin 

 places in their closing membrane, and are then spoken of as compound 

 pits. A special example of such pits is afforded by the sieve-pits, in 



Fig. S7. — Part of transverse section of a stem of Impatiens 

 parmjlora. c, Epidermis ; c, collenchyma ; p, thin- walled 

 parenchymatous cells ; i, intercellular space, (x 800.) 



