10 APPARATUS AND METHODS. 



purposes it is sufficient to use either strong alcohol or 

 formalin, or (better) one of the acid fluids. (1) Formalin 

 is useful for both fixing and preserving ; simply put the 

 material into a 4 per cent, solution formalin as sold is a 

 40 per cent, solution and must therefore be mixed with 

 eight or nine times its volume of water and keep it there 

 until needed for use, when it should be rinsed in water, since 

 the formalin fumes are irritating to eyes and nose. (2) Put 

 the material for 24 hours in 300 c.c. of water containing 

 2 grams chromic acid and 3 c.c. glacial acetic acid ; wash 

 in running water, or in a vessel of water changed fre- 

 quently, for about two hours ; then place for a day in suc- 

 cession in 30 per cent., 50 per cent., and 70 per cent, 

 alcohol ; and finally preserve in strong methylated spirit 

 (= about 95 per cent, alcohol). 



Objects like filamentous Algae, Mosses, Liverworts, 

 Fern prothalli, root-tips, etc., may be placed entire in the 

 fixing and preserving fluids, but larger specimens should 

 be cut into pieces about 1 cubic centimetre in size. 



9. Section Cutting. In examining the structure of a 

 solid mass e.g. a stem, root, or leaf we can learn a good 

 deal by crushing, teasing, or macerating the tissues, but 

 these simple methods should be supplemented by the 

 preparation of thin sections cut in different directions. 

 Instructions as to the direction in which sections should be 

 cut are given in connection with the various types. It is 

 only necessary to remember that for the complete study of 

 a solid cell-mass, it is necessary to cut sections in three 

 planes at right angles to each other. 



For instance, three sets of sections are required in the 

 case of a cylindrical stem: (1) transverse, exactly at 

 right angles to the long axis ; (2) radial longitudinal, 

 including the long axis ; (3) tangential longitudinal, 

 parallel to a radial plane but not including the axis. 

 Obviously, in a cylindrical organ it will be only the 

 central part of the tangential section that will give the 

 desired plane at each side of the section the radii will be 

 cut obliquely and not at right angles. 



In most cases it is necessary to keep both the razor and 



