38 Botanical Microtechnique 



oriented, move the boat lo the cold end of the plate. Sweep the 

 Biinsen flame rapidly over the surface of the paraffin; this permits 

 contraction on the upper surface while the bottom of the paraf- 

 fin is cooling, thereby preventing the formation of cavities. As soon 

 as the paraffin is hardened enough to keep the pieces of tissue from 

 moving, float the boat in a pan of cold water and brush with the 

 Bunsen flame again. Allow the surface to solidify, and submerge the 

 boat in the water, holding it under with a staining jar cover or other 

 weight. When the paraffin is thoroughly cooled, peel off and discard 

 the paper boat. 



Some purchases of paraffin wax, and occasional lots of proprietary 

 compounded waxes, develop white areas or bubbles if kept at room 

 temperature after casting. This can be minimized or prevented by 

 storing the cast blocks in a refrigerator for several days. If the sjx)ts 

 appear upon subsequent exposure to room temperature, permanent 

 low temperature storage should be provided. 



When using the warm-pan method (Fig. 5.2 B) . place a paraffin- 

 soaked boat on the triangle, and warm the bottf)m of the pan Avith 

 a Bunsen flame of such size that the paraffin on the boat is kept just 

 melted. Pour the material into the boat, and arrange the pieces, oc- 

 casionally flaming the top surface of the paraffin. There is little 

 danger of overheating the material; hence the operator need not 

 hurry in arranging the pieces. When the pieces are arranged satis- 

 factorily, remove the burner, and jjoiu" cold water into the ])an tmtil 

 the water level is slightly above the bottom of the boat. The pieces 

 become liardened into place quickly. Sweep the flame o\er the stnface 

 of the paraffin to permit internal contiaction. (louiplete (lie hardening 

 as in the former method. 



The sj:)acing of pieces in the block dej)cncls on the si/e of the 

 pieces. Root tips and small pieces of leaf can be sj)aced 5 iinii. apart; 

 large pieces of stem require more supporting ])arallni dining section- 

 ing and should therefore be spaced at least 1 (in. aj)ait. Very slender 

 loot tips, coniferous needles, and similai oljjc'cts may be blocked 

 in groups of three or more j)ie(es laid parallel so thai they can be 

 microtomed simultaneously. 



RECASTING 



Poor paraffin of a cast block can be replaced with good paraffin, 

 or j)oorly arranged material can be rearranged and recast, and exces- 

 sively large pieces can be trinnned and recast. Cut the jiieces out of 

 the block, trim the pieces if desired, cut away excess paraffin if it is 



