MICROTOME. The microtome is an in strument used by botanists and zoologists for cutting thin sections of plant or animal tissues. A sharp and highly tempered knife is held in a rigid clamp, while the object to be sectioned, also firmly held in a clamp, is moved up, a little at a time, as successive sections are cut. To one unacquainted with modern microscop ical technic, the results obtained with the latest microtomes seem almost miraculous. The pollen grain of a lily is so small that it is barely visible to the naked eye, hut it can he cut into 50 sections, which can be mounted in order without the loss of a single section. A photo micrograph of such a section can be made, and lantern slide made from the photomicrograph, and the section of the pollen grain may be pro jected so as to cover a 15-foot screen, its thou sands of starch grains appearing as large as walnuts and its two nuclei as large as footballs. Sections as thin as 25,000 to the inch have been cut successfully. Such thin sections, however, are hardly necessary in practical work. Most sections are cut at about 10 times that thick ness, or 2,500 to the inch. Before such thin sections can be cut, material must undergo a rather complicated treatment. A root tip, for instance, may be treated as follows: It is killed and somewhat hardened in chromic acid; the acid is washed out in water; the root tips are then placed in weak alcohol, then in stronger and stronger alcohol, several hours in each solution; then in absolute alcohol which not only completes the hardening but removes the last traces of water; then some oil, like xylol, is mixed with the absolute alcohol in succes sively stronger solutions until pure xylol is reached; paraffin is then added gradually for several days; the material is then placed in a paraffin oven and thoroughly infiltrated with paraffin, after which it is poured out and cooled, so that the root tips are now embedded in a matrix of paraffin. A rectangular block
containing a root tip is now fastened in the microtome and sections are cut. As each suc cessive section is cut, it sticks to the preceding section, so that a ribbon, often several feet in length, may be obtained. Thus the sections are in perfect series. The sections are then mounted on glass slides, the paraffin is dissolved away, various stains are applied and finally a drop of balsam and a thin glass cover complete the preparation, and it is ready for use. Inves tigations upon the structure of tissues and cells are dependent upon the microtome and micro scope and histological technic. Consult Cham berlain, C. J., 'Methods in Plant Histology' ; Guyer, M. F., 'Animal Micrology.'