204 BOTANICAL MICROTECHNIQUE 



it be at all sweet there is chloroform still present. The chloroform must be 

 entirely driven off before imbedding, otherwise the paraffin will not cut well. 



Material in xylol and paraffin must be treated differently from that in 

 chloroform. Xylol cannot be removed easily by heat. Consequently the ma- 

 terial must be transferred through solutions with less xylol in them until it is 

 carried into pure paraffin. The simplest way is to pour off solutions and add 

 melted paraffin, keeping the vials in the bath. The mixtures of paraffin and 

 xylol may be saved and used again or simply left in the bath to gradually 

 purify as the xylol is driven off. Finally the material is placed in two or 

 three changes of pure paraffin to remove the last trace of xylol. It will do 

 the material no harm to remain several days in the mixtures of paraffin and 

 xylol, and structures with thick walls or coats (such as the megaspores of 

 Selaginella) must be left sometimes for weeks before infiltration is completed. 



193. Imbedding. The material is now in pure melted paraffin and ready 

 to be cast in a cake. Most subjects can be cut in paraffin, which melts at a 

 relatively low temperature, 50°-52° C. Others require a hard paraffin with 

 a melting point of 56° or higher. In general it is better to imbed in a medium 

 paraffin and plan to cut in a room at a cool temperature. 



Petri dishes are good receptacles for the casting, or paper trays may be 

 used. Two L-shaped pieces of metal on a glass plate are convenient, since 

 the size of the mold may be readily adjusted to the object. The interior of 

 the receptacle should be smeared with glycerin to prevent the paraffin from 

 sticking. The melted paraffin is poured into the receptacle with the material 

 and the latter is then arranged with a heated needle. Finally the receptacle 

 is gently lowered into a vessel of cold water, so that the paraffin is cooled 

 quickly, which prevents its crystallizing, but it cannot be entirely immersed 

 until the paraffin has solidified over the top. When cold, the cake may be 

 cut up into blocks of convenient size which are ready for cutting (Sec. 196). 

 Material that is perfectly imbedded will be preserved indefinitely in a form 

 that gives no further trouble, and for this reason it is often desirable to run 

 material into paraffin instead of keeping it in alcohol. 



SECTIONING 



194. Free-hand sectioning. Free-hand sections are, as a rule, sufficiently 

 satisfactory for general studies of the tissues of spermatophytes and pterido- 

 phytes. The technique is as follows. The object is held between the thumb 

 and finger of the left hand, or, if small or soft, it must be placed between two 

 flat pieces of pith. The razor is held in the right hand and is drawn across 

 the object with the edge towards the operator and the blade sliding on the 

 forefinger of the left hand. There should be water on the upper edge of the 



