30 SIMPLE PITS 



layers formed by the protoplast of one cell, coincide with the 

 thin areas on the outer sides of its walls, where the thickening 

 has been deposited by the protoplasts of the surrounding cells. 

 At these points, known as simple pits, the original middle lamella 

 (the pit-mcnihrane, Fig. 14, A, pm.) alone separates the adjacent 

 cells. In these cases the bulk of the protoplasmic connections 

 (cf. p. 24) pass through the pit-membrane. 



Good examples of simple pits are seen in sections of Elder 

 pith (Fig. 14, B). In surface view they appear as irregularly 

 scattered oval or elliptical areas which are more transparent 



Fig. 14. — Thickened cell-walls. A, Portion of the endosperm of the Date 

 in section. B, Pith-cell of the Elder. C, A group of stone-cells of 

 the Pear, together with a small part of the adjacent flesh. D, Two 

 isolated stone-cells, m.l., middle lamella; p., pits seen in section ; 

 pm., pit-membrane ; Ps., pits in surface view ; s.c, stone-cells. 



than the rest [Ps.). In section (/>.) the walls appear broken at 

 sundry points ; but careful focussing shows that the apparent 

 gaps are really bridged by a thin line, the pit-membrane. In 

 certain cases [e.g. the endosperm of the Date (Fig. 14, A), the 

 cotyledon-walls of the Lupine and many other Leguminosffi, and 

 cells of the cortex of the Mistletoe] the thickening is so extreme 

 that the cavity of the cell becomes much reduced, and the pits 

 then appear as deep depressions within the wall. It is in such 

 tissues that the protoplasmic connections between cell and cell 

 can most readily be demonstrated. 



