Plant histology: Paleobotany

Paleobotany has made such rapid progress during the last ten years that scarcely any problem involving the anatomy of living vascular plants can be investigated intelligently without some knowledge of Mesozoic and Paleozoic forms. Material, especially that of Paleozoic Pteridophytes and Gymnosperms, is becoming available, and consequently it is increasingly necessary for laboratories to have apparatus and technic for cutting rock-sections.

The outline of the process of cutting a rock-section is very simple:

1. Saw the rock into two pieces. 2. Polish the cut surface. 3. Fasten the cut surface to a piece of glass with hot shellac. 4. With the saw, make another cut, as close to the glass as possible, so as to leave a thin section firmly fastened to the glass. 5. Grind and polish until the section is as thin as possible, or as thin as you want it. 6. Wash all polishing powder off with water. 7. Dry completely and, either with or without moistening in xylol, mount in balsam.

A word of suggestion in regard to these various points may not be amiss.

1. Most rock-sections are cut with a rather expensive and quite complicated instrument, called a "petrotome." The saw is of the circular type, is made of tin or other soft metal, has no teeth, but has diamond dust driven into the margin. A rigid clamp holds the object, and the saw, constantly cooled by a stream of water, gradually cuts through the specimen. If the piece to be cut is more than 5 or 6 cm. in diameter, a band saw is better; and if the piece is 10 or 20 cm. in diameter, the band saw is necessary.

2. The cut surface is most easily polished on a revolving brass plate, kept wet and liberally powdered with fine carborundum. When the surface has become even and smooth, the specimen is ready for the next step.

3. Fasten the polished surface to the glass slide upon which the section is to be mounted. Plate glass 3 or 4 mm. thick is best for sections larger than 3 or 4 mm. square. Gradually heat the slide until it is quite hot. Melt upon the slide the thin flakes of white shellac used by painters; heat the object and press the polished surface very firmly into the melted shellac. Canada balsam, from which the xylol has been driven off by heating, can be used instead of shellac. Much of the Paleozoic material is in the form of coal balls. After the ball has been cut in two, it is often difficult to hold the hemispherical piece in a clamp, especially if the piece is small. In such cases, it is better to fasten the polished surface to a convenient piece of marble, about 2.5 cm. thick, and 5 or 6 cm. square. The marble is easily held in the clamp. As soon as the slide, or marble, and object are cool, the next cut can be made.

4. Fasten the object in the clamp and saw as close to the glass, or marble, as possible, thus leaving a thin section cemented to the slide or marble. If marble has been used, the section is removed by heating or by dissolving it off with xylol. It can then be fastened to the glass slide for grinding and polishing. Anyone who can handle tools should soon be able to cut a section 1 mm. thick. A skilled technician can cut sections as thin as 0.5 mm.

5. The second grinding must be very careful and accurate. Do the polishing on the revolving disk. The glass slide allows one to note how the process is progressing.

6. When the section becomes thin enough, or even before if it begins to crack, wash off the powder.

7. It is usually a good plan to use rather thick balsam for mounting, even if it should be necessary to heat it a little to make it flow well.

By this method, sections of fossils 10 * 15 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 examination with a 2 mm. oil immersion lens.

Of course, this method can be used for such objects as walnut and hickory shells.

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