14 HISTOLOGY 



MITOSIS. 



Mitosis, also called indirect division and karyokinesis, is the ordinary 

 mode of cell division. Although it is a continuous process, it has been 

 conveniently divided into four successive phases the prophase, metaphase, 

 anaphase, and telophase. During the prophase the chromatic material 

 of the nucleus prepares for division and collects in the center of the cell. 

 It is divided in halves in the metaphase, and the two halves move apart 

 during the anaphase. The chromatic material becomes reconstructed 

 into resting nuclei during the telophase. The various patterns which the 

 chromatic material and protoplasmic fibrils present during these phases 

 are known as mitotic figures. 



Mitotic figures are found in all rapidly growing tissues, but especially 

 favorable for preliminary study are the large cells in the root tips of plants. 

 In longitudinal sections of root tips, the cells are cut at right angles to 

 the plane of cell division, which is desirable; and often hi a single section 

 5 mm. long, all the fundamental stages may be quickly located. The 

 ' following general description of mitosis is based upon such easily ob- 

 tained preparations, and the plant selected is the spiderwort (Trade- 

 scantia mrginiana). 1 They may be satisfactorily stained with saffranin, 

 or with iron haematoxylin and a counter stain such as orange G. There 

 are many descriptions of mitosis in root tips, among them the following: 



Rosen, (Hyacinthus orientalis) Beitr. zur Biol. der Pflanzen, 1895, 

 vol. 7, pp. 225-312; Nemec, (Allium cepd) Sitz.-ber. kon. Ges. der Wiss. 

 Prag, 1897, No. 33, pp. 25-26, and Jahrb. fur wiss. Bot., 1899, vol. 33, pp. 

 313-336; Schaffner, (Allium cepa) Bot. Gaz., 1898, vol. 26, pp. 225-238; 

 Hof, (Ephedra major) Bot. Centralbl., 1898, vol. 76, pp. 63-69, 113-118, 

 166-171, 221-226; Gregoire and Wygaerts, (Trillium grandiflorum) La 

 Cellule, 1904, vol. 21, pp. 1-76; Farmer and Shove, (Tradescentia mrgin- 

 iana) Quart. Journ. Micr. Sci., 1905, vol. 48, pp. 559-569; Richards, 

 (Podophyllum peltatum) Kansas Univ. Sci. Bull., 1909, vol. 5, p. 87-93. 



The cells to be described are found in the interior of the root tip, just 

 back of the protecting cap of cells which covers its extremity. They are 

 oblong in shape and their long axis corresponds with that of the root. 

 The walls are very distinct, and the cells consist of granular vacuolated 

 protoplasm, which in preserved specimens is generally irregularly shrunken. 



The resting cells (Fig. 12, A) contain large round nuclei in which the 

 chromatin is in the form of fine granules evenly distributed throughout 

 the nucleus. A nucleus usually contains from two to five round nucleoli, 

 each of which, when in focus, is seen to be surrounded by a clear zone. 

 The nuclear membrane is distinct. 



1 Good specimens may be obtained from any rapidly growing root tip. Those starting from 

 hyacinth bulbs placed in water are very favorable. Onion root tips have been extensively 

 used, and also those of bean and corn seedlings. The pointed ends are snipped off and dropped 

 into Flemming's stronger solution. 



