86 Botanical Microtechnique 



(Fig. 8.2) , fasten into the microtome damp, and adjust the universal 

 joint until the desired plane of sectioning is parallel to the plane of 

 travel of the knife (Fig. 6.3) . Keep the tissues moistened with glycerin- 

 alcohol. If the upper surface of the material is not level, trim with 

 a razor blade, sparing the microtome knife from rough trimming 

 work. The best cutting angle for the knife-edge, with reference to the 

 line of travel, ranges from 30 to 40°. The vertical tilt or declination 

 of the flat side of the knife is also subject to variation. Begin with 

 just enough tilt to enable the back of the ground wedge to clear 

 the tissues (Fig. 6.3 A, B) . Bring the tissues into cutting contact Avith 

 the knife, using the hand-operated feed, making each vertical feed 

 movement ajter the knife has passed over the material on the return 

 stroke. In order to avoid damaging the knife-edge, feed in steps of not 

 over 15 |i. Make sure that there will be ample clearance between 

 the knife carriage and the tissue carrier e\'en after many sections have 

 been cut. 



When each stroke cuts a complete section, set the thickness gauge 

 and the automatic feed device. A thickness of 15 to 20 |j, is satisfactory 

 for most woody subjects. Keep the material and the knife flooded with 

 95% alcohol while cutting sections, and transfer each section as soon 

 as it is cut to 95% alcohol in a watch glass or other shallow container. 

 Vary the cutting angle and declination until sections slide up onto 

 the knife without compression, curling, or breaking. Newly embedded 

 material is liable to be hard and brittle and to curl. Curling can be 

 minimized by holding a finger, moistened with alcohol, in light con- 

 tact with the material during the cutting stroke, until the knife has 

 cut through the marginal celloidin and enters the material. 11 

 scratches are evident on the cut surface, the portion of the knife being 

 used may have bad nicks. Shift the knife longitudinally in its clamp 

 and discard the next few sections. Sections can be stained at once, or 

 they may be stored in glvcerin-alcohol indefinitclv. In the case of 

 materials that do not (inl. it is possible to cut sc\cral hundred 

 sections, store them in a bottle of glycerin-alcohol, and remove as 

 many as needed for staining at any time. 



Some materials (an be cut readilv enough hut dilliculties arise 

 after sectioning. Fhe sections max cuil soon alter rcinoxal iiom the 

 knife and become increasingly tightlv ciuled during staining and 

 dehydration. Being made l)iitilc 1)\ dehydration and clearing, the 

 sections break when an attempt is made to uncurl ihem for mounting. 

 The following method is usually effective for such material. Keeping 

 the- knife well floodccl with alcohol, cut a section. Hold a finger under 



