THICKENED WALLS PITS. 



73 



razor the thinnest possible section. This section (Fig. 26) shows 

 us cells of approximately rectangular form. The walls of these 

 cells are strongly thickened, the thick- 

 ening layer being, however, pierced by 

 numerous simple pits. If the section 

 has skirted a cell- wall so that it offers 

 a surface view, the pits appear as round 

 pores (m), as can be seen in the upper 

 cell of the adjoining figure. From the 

 side the pits appear as canals, which 

 extend from the cell-cavity up to the 

 primary cell-wall. The pits of adjoining 

 cells correspond, and are separated by 

 the primary wall (p), which we shall 

 designate the closing membrane. The 

 inner surface of the thickening layer is 



distinguished by stronger refractiveness ; ^ 2 6.-From the endosperm 



it forms the limiting layer. We now of&niith(jalnmumbeUatum. m, 



. . , . -pits seen from above ; , closing 



lay some sections in iodine, remove membrane in pits seen in profile ; 



any superfluity, and run in a drop of " nucleus ( x 240) - 

 dilute sulphuric acid. The thickening layers of the cells swell, 

 and at the same time colour bright blue. Between these swelling 

 thickening layers very delicate unswollen walls remain, and in 

 thin parts of the section it can be determined that they have 

 taken on a brownish tone. They are the so-called middle 

 lame Use, the walls which existed in the cells prior to the com- 

 mencement of the thickening, and which also traversed the 

 closing membrane of the pits. In chlorzinc iodine solution the 

 thickening layers of these cells colour dirty violet, and swell the 

 middle lamellae again becoming brownish. In order to follow 

 this reaction well it is desirable to put a dry section, without 

 wetting in water, direct into the chlorzinc iodine. The cells are 

 densely filled with protoplasm and granular reserve food-mate- 

 rials, the whole of which become yellow with chlorzinc iodine. 

 In each cell the nucleus can be readily disclosed by the aid of 

 acetic methyl green. 



As an alternative to this material, and offering very much the 

 same characters, we may take the cortex of young twigs of the 

 Cherry-laurel (Cerasus Lauro-Cerasus). 



Thickened Walls in Date-stone. The thickening layers of the 

 cells in the endosperm of the seed or " stone" of the Date-palm 



