SECTIONING 205 



razor, and as the sections are cut they should slip into the water and float 

 in it. When a number of sections have been cut, they may be removed with 

 a brush to a watch glass of water. It is, of course, impossible to cut good 

 sections with a dull razor. 



A small hand or table microtome is frequently of great assistance, taking 

 the place of free-hand sectioning. The object is held between pith in an 

 adjustable clamp, and the razor slides over a glass plate. A large number of 

 sections, sufficient to supply a class, may thus be easily cut from such an 

 object as a piece of stem or leaf. 



Some methods of staining free-hand sections have been outlined in Sees. 

 183-186. Those of preserved material in alcohol require no further treatment 

 before being placed in the stain, but sections of living material must be fixed 

 before they can be satisfactorily stained. If the tissue is firm, the simplest 

 method is to place the sections directly into absolute alcohol, when after an 

 hour or so they may be stained. If, however, the tissue is delicate, or the cells 

 contain much protoplasm, it is best to fix in medium chrom-aoetic acid (Sec. 172) 

 for two to twelve hours, washing for an hour or more in several changes of 

 water. Such sections may be stained at once or run up into alcohol. 



195. Sectioning in celloidin. As previously stated (Sec. 190), very firm and 

 hard tissues such as characterize the sporophyte generation of pterido- 

 phytes and spermatophytes cannot be cut in paraiBn. Exact work is fre- 

 quently only possible through sections cut in celloidin. Furthermore, large 

 sections of stems, roots, etc., can only be cut by this method. The tech- 

 nique is, however, somewhat long, and for the purposes of general studies 

 free-hand sections, or those cut on a hand microtome, are likely to prove 

 sufficiently satisfactory. A detailed account of the celloidin method as era- 

 ployed in botany is given by Plowman, Botanical Gazette, Vol. XXXVII, 

 p. 456, 1904 ; and in Chamberlain's Methods of Histology. 



196. Sectioning in paraffin. Sectioning in paraffin is only possible for 

 structures of reasonably soft tissue and not very large. The advantages of 

 the method are that the sections may be cut much thinner than in celloidin 

 or free-hand, that they may easily be arranged serially, and that they may 

 be stained with greater precision. The method is very generally applicable 

 throughout the thallophytes and bryophytes and for the gametophyte gen- 

 eration of the pteridophytes and spermatophytes, together with the softer 

 tissues of many organs and developing structures of the sporophyte genera- 

 tion of the latter groups. 



Paraffin material is cut on a microtome. The most convenient instru- 

 ment is the rotary microtome of the Minot type, of which there are several 

 forms on the market. The sliding microtome of the Jung-Thoma type is 

 also excellent, and while not so rapid as the rotary is sometimes more accu- 

 rate for the most exact work. 



