204 H. von Mohl on the Investigation of Vegetable Tissue 



verse section of a cylindrical cell, because the inner and outer 

 layers of the cell- wall have not the same accurate concentric 

 arrangement as in the latter. 



The transverse section of a thin-walled tissue formed of poly- 

 hedral cells, in which the cut surfaces appear in the form of 

 straight lines, presents an appearance deviating in many respects. 

 In such a section a more or less considerable number of the cell- 

 walls are placed perpendicularly to one of the two Nicols ; these 

 consequently remain totally invisible, while the remainder of the 

 walls, standing obliquely as regards the Nicols, appear uniformly 

 bright in their whole length. From the apparent deficiency of 

 part of the cell-wall, the figure acquires an unconnected appear- 

 ance, and looks as if it were a torn and imperfect preparation. 

 But if, while looking into the microscope, the object is rotated 

 in a horizontal direction, the previously invisible cell-walls emerge 

 from the darkness, while others previously illuminated become 

 invisible. 



To see these phsenomena in their full beauty, choice should be 

 made of a thin transverse section of a tissue composed of more 

 or less elongated thin- walled cells, e.g. of the stem of herbaceous 

 Monocotyledons, or of the wood of JEschi/nomene paludosa : the 

 transverse section of short polyhedral cells, e. g. of Elder pith, is 

 less advantageous, because in this, horizontal walls of cells and 

 obliquely-directed side- walls come into view in many places in 

 company with the horizontal external surfaces of the side-walls, 

 which produces a complication of the phsenomena. In making 

 the preparation, two points must be borne in mind. In the first 

 place, the section must be made thinner in proportion as the 

 substance of the object acts more strongly upon polarized light. 

 The best image is obtained when the preparation appears with 

 bright white light : if the section be too thick, it presents more 

 or less of prismatic colouring in its different parts, which is 

 unfavourable in many investigations. The second precaution, 

 which must never be lost sight of, relates to the fluid in which 

 the object is kept. Most vegetable substances refract light far 

 more strongly than water : in the same degree as this circum- 

 stance is favourable in ordinary microscopic investigations, is it 

 unfavourable in researches with the polarizing microscope. In 

 this the best image is obtained when the refractive power of the 

 preservative fluid stands as near as possible to that of the object. 

 Hence, when the nature of the object allows, as is the case with 

 cellulose membranes, starch-granules, &c., it should not be exa- 

 mined in water, but in essential oil, for instance oil of turpentine, 

 or mounted in Canada balsam or a similar resinous substance. 

 The more transparent the preparation becomes, the more beau- 

 tiful is the image that it gives. 



