128 Methods in Plant Histology 



7. It is usually a good plan to use rather thick balsam for mount- 

 ing, even if it should be necessary to heat it a little to make it flow 

 well. 



By this method, sections of silicious fossils 10X15 cm. have been 

 cut thin enough for examination with a 4 mm. objective. Sections 

 3 or 4 mm. square have been cut thin enough for satisfactory examina- 

 tion with a 2 mm. oil immersion lens. 



Of course, this method can be used for such objects as walnut 

 and hickory shells. 



THICK SECTIONS 



It is sometimes desirable to make very thick sections to show 

 general topography rather than detail. A longitudinal section of 

 the fully grown ovule of Ginkgo or a cycad may be cut as thick as 

 3 to 5 mm. so as to include the entire group of archegonia. A slab 

 can be cut from each side of the ovule with a fine saw, and a razor 

 can be used for smoothing. If the section is from fresh material it 

 should be fixed, washed, etc., with about the same periods as if it 

 were to be imbedded in paraffin. When thoroughly cleared in xylol, 

 the section should be put into a flat museum jar of suitable size and 

 kept in xylol. Even before the stony coat of a cycad becomes too 

 hard to be cut with a razor such thick sections are very instructive. 

 Stain very lightly, or not at all. Sections of Zamia or other cycad 

 stems, about 2 mm. thick, make instructive mounts, since they show 

 the peculiar course of the bundles, a feature which is largely lost in 

 thin sections. 



LAND'S GELATIN METHOD 



It is sometimes desirable to get sections of partly disorganized 

 material. A matrix is necessary to hold the parts in place, but 

 dehydration may make the tissue unnecessarily hard to cut. 



Soak ordinary gelatin (which can be obtained at the grocery) in 

 water until no more is taken up. Then drain off the excess water 

 and liquefy the gelatin by heating. Place the material previously 

 soaked in water in the melted gelatin and keep it there for several 

 hours. Place also in the gelatin some small blocks of hard wood 

 to serve as supports in the microtome. The material to be sectioned 



