Microtome Sectioning of Material in Paraffin 49 



able portions of interwoven spongy parenchyma. In such thick sections 

 it is difficuh to ascertain the limits of individual cells and the true 

 location of bodies within the cells or myceliimi in the tissues. A section 

 5 \i thick would include adequate longitudinal slices of palisade cells, 

 but the sections cut from various portions of the irregular spongy 

 cells would appear as separated fragments (Fig. 6.5 C) . Sections of 

 approximately 10 ^ might be a good compromise by showing enough 

 of the spongy cells to indicate continuity of contact. 



When cutting tissues infected with a filamentous fiuigus, it is 

 often necessary to make some apparently excessively thick sections in 

 order to include sufficiently long strands of the mycelium. Lily ovules 



- ZOp — I 



Fig. 6.5— Method of ascertaining the appropriate thici<.ness at which sections should 

 be cut: A, perspective view of leaf tissues; B, respective numbers of cell layers in- 

 cluded in sections of different thickness; C, disjointed appearance of leaf tissues 



in excessively thin sections. 



in the four- to eight-nucleate stage must be cut 15 to 20 \x thick to 

 include the complete set of embryo-sac nuclei in a sufficiently high 

 ratio of slides. Onion root-tip slides of 8 to 12 |i include enough 

 chromosomes of the complement to show their approximate number, 

 but it is desirable to have some slides of 4 to fi [i to show chromosome 

 details for the more inquisitive elementary students, and for advanced 

 courses. 



The optimum thickness of sections is obviously a compromise 

 between transparency and clearness of separate structures, and the 

 preservation of the relationship and continuity of associated struc- 



