94<5 APPENDIX. BOOK I. 



the chromatin fibrillae takes place, but he considers that two groups of fibrillae are formed 

 which travel to the poles of the spindle; (3) he is of opinion that the equatorial plate 

 consists of chromatin and the rest of the spindle of achromatin, whereas Strasburger 

 states that the equatorial plate consists of the whole of the nuclear substance (both 

 chromatin and achromatin) and that the spindle consists of cell-protoplasm ; (4) Flem- 

 ming does not admit that the fibrillae, derived from the equatorial plate, which travel to 

 each pole, undergo fusion to form a new nucleus. (See Quart. Journ. Micros. Sci. 1882.) 



The illustrative cases of cell-division which have been given in the text refer only to 

 cells which contain a single nucleus. In cells which are or are about to become multi- 

 nuclear, — in cases, that is, in which nuclear division is not followed by cell division,— 

 the process of nuclear division is usually simpler. In the older internodal cells of the 

 Characeae, in older parenchymatous cells of Lycopodium and of some Phanerogams 

 {Taraxacum, Glyceria, Sempervl'vum, Cereus, Solarium^ etc.), and occasionally in Falonia (in 

 all of which cases the cells become multinuclear), the nucleus simply divides by con- 

 striction, the chromatin granules being shared equally between the two new nuclei 

 without any indications of karyokinesis. Division may begin again in the two new 

 nuclei even before they are separated from each other. This process of nuclear division 

 has been termed fragmentation. In Falonia and in Codium a rudimentary form of karyo- 

 kinesis has been observed by Schmitz and by Berthold (Mittheil. der zool. Stat, zu 

 Neapel, II, 1880), which appears to occur commonly among Thallophytes. In such a case 

 the nucleus becomes elongated ; its ends enlarge, whereas the middle part remains narrow ; 

 the ground-substance of the nucleus now presents a faint longitudinal striation, and the 

 chromatin-granules either simply travel to the two ends of the nucleus, or become rod- 

 shaped and aggregate in the equatorial plane to form a rudimentary nuclear disc, which 

 splits in the ordinary way, each half travelling to one end of the nucleus. The narrow 

 middle portion now undergoes absorption, and the two ends round themselves off to con- 

 stitute two new nuclei. Treub (Sur des cellules v^g^tales ^ plusieurs noyaux. Arch. 

 N^erland., XV, 1880) has observed the division of the nucleus in the multinuclear bast- 

 fibres and laticiferous cells of various Phanerogams, and finds that it takes place in the 

 manner described in the text for uninuclear cells, except that no cell-plate is formed. 

 See also Johow, Die Zellkerne von Cbara, Bot. Zeitg. 1881. 



An illustration of the independence of cell-division with regard to nuclear division 

 is afforded by Cladophora. The cells of this plant are multinuclear, and Strasburger has 

 found that the division of the nuclei takes place in the manner observed by Treub in 

 bast-fibres and laticiferous cells : the division of the cells bears no relation, either in 

 time or space, to that of the nuclei ; the new cell-wall is formed in much the same 

 manner as in Spirogyra. 



Page 19. Schmitz has come to the conclusion {loc. cit. 1880) that the cell-wall is 

 formed by the actual conversion of a peripheral layer of the protoplasm into cellulose. 

 He is also of opinion that stratified cell-walls are formed by the deposition, one within 

 the other, of successive layers, and not by intussusceptive growth with subsequent differ- 

 entiation. These views are also held by Strasburger ; Bau und Wachsthum der Zell- 

 haute, 1882 (see p. 960). 



Both Schmitz and Strasburger hold that the surface-growth of cell-walls is not due 

 to the intercalation of new solid particles (intussusception), but is simply the expression 

 of the stretching of the cell-wall by the cell-contents. 



Frommann (Protoplasma, 1880) believes that he has been able to trace a connexion 

 of the protoplasm through the walls of adjacent cells. That protoplasm can pass through 

 closed cell-walls is beyond doubt. See Strasburger, loc. cit. p. 247. 



Page 23. The formation of bordered pits. A very different account from that in 

 the text is given by Mikosch (Unters. lib. Entstehung und Bau der Hoftiipfel, Sitzber. d. 

 k. k. Akad. in Wien, LXXXIV, 1881). Sachs has found (Ueb. die Porositat des Holzes, 

 Arb. d. bot. Inst, in Wiirzburg, II, 1879) that in the spring-wood at least the bordered pits 

 are closed by a membrane. See also Strasburger, Bau und Wachsth. d. Zellhaute, 1882. 



