74 V. CELL-WALL, AND CELL CONTENTS. 



(Phwnix dactylifera) have a very similar appearance ; the cells,, 

 however, are more elongated, their cavity narrower, the walls 

 proportionally much thicker, and they are radially arranged. 

 Cross and longitudinal sections, therefore, provided they corre- 

 spond with the radii, show the cells in longitudinal view, while 

 tangential sections, which cut the radii at right angles, show 

 cross-sections of the cells. Here also iodine and sulphuric acid 

 colour the thickening layers blue, chlorzinc iodine solution a very 

 beautiful violet, and by slow swelling numerous lamellae are 

 usually brought into view. 



" Bordered Pits " of Pinus. We turn now to the pine-wood 

 (Pinus, etc., any species, preferably P. sylvestris, the Scotch fir), 

 in order to study bordered pits. For this purpose we take a piece 

 of wood, either dry, or, better still, preserved in alcohol, from a stem 

 as old as possible. First we prepare with a sharp pocket-knife the 

 suitable surfaces for cutting one radial, parallel to the long axis of 

 the stem, one tangential to the same, and one directed perpendicu- 

 larly to this axis. The concentric yearly rings which are visible 

 with the naked eye upon every piece of pine -wood will provide 

 us with the necessary bases from which to decide as to the 

 directions in question, and the orientation of the sections must be 

 very carefully attended to. The radial longitudinal section cuts 

 the yearly rings perpendicularly ; the tangential longitudinal 

 section is so much the more perfect, the more parallel it runs to 

 the yearly rings. The cross-section is directed perpendicularly 

 to both longitudinal sections. In the following preparation of 

 microscopical sections, in order that the sections shall be good, 

 and not to damage the razor, quite special precautionary rules 

 must be adopted. If the razor is hollow-ground, rightly directed 

 sections can be taken only from the edges of the piece of wood, 

 'i.e., so long as the back of the razor does not yet rest upon the 

 cut surface. However, in general, only slightly hollowed razors 

 should be used for cutting wood, as those greatly hollowed easily 

 " give " ; and indeed it is recommended to use razors which are 

 ground flat on one side, i.e., the side which will rest upon the 

 cut surface, though these razors have the disadvantage that 

 they are not easily sharpened. The cut surface must always be 

 moistened ; the sections must be as thin as possible. It is not 

 necessary to have them of any size. A section which appears to 

 be too thick should not be cut to the end ; it is better to with- 

 draw the razor from the cut in order not to notch the edge. 



