43§ RUDOLF BEER [december 



cells is a smaller, shrunken, and more solid body, provided with a 

 comparatively small nucleolus. One can well understand how those 

 internal differentiations, which we have described for the young nucleus, 

 could not readily take place in the dense body of the more aged 

 nucleus. In consequence there would be no line of weakness formed 

 which would sharply and cleanly break across under the influence of 

 the tension impelling division ; instead, the dense nuclear substance 

 would be drawn out and variously contorted at the line of division. 



The nuclear differentiation, accompanied by a constriction which is 

 so seldom found, marks the link between the fragmentation in the 

 younger and older cells. In this case the nuclear substance, whilst 

 not being too dense to allow the internal changes to take place, 

 has yet become, even in the granular substance, too firm for a clean 

 break to be formed between the daughter nuclei. 



Judging from these observations on Zca Mays, the vexed question 

 whether nucleolar division does or does not always precede nuclear 

 fragmentation, is not one to be answered in a sentence. The age, the 

 general density of the nucleus in that particular tissue or plant, the 

 intensity of the impulse to divide, all have to be carefully considered. 

 The discordant statements made on this subject by eminently careful 

 observers are to be explained by their having examined the same 

 tissues at different ages or grown under different conditions (cf. 6, 11, 

 and 17). 



Before leaving the subject it should be mentioned that these older 

 nuclei which are fragmenting by constriction never show karyokinetic 

 figures, and have apparently lost the power of dividing indirectly. 



As has been mentioned, the above observations were made on the 

 leaf-sheaths and root-apices of Zca Mays. In the latter the directly 

 and indirectly dividing nuclei could be found in adjoining cells, as was 

 the case in the leaf-sheaths. Multinuclear cells of a similar appearance 

 have also been observed in the stem of Zca, in the leaf-sheaths of 

 Sccale cerealc (rye), the leaf-sheaths and young stems of Triticwn 

 vulgare (wheat), the leaf-sheaths of Hordeum sativum (barley), and 

 Dactylis glomcrata (cock's-foot grass). 



In conclusion, 1 should mention that the best results were obtained 

 from young seedlings of the plants mentioned ; plants of Zea, a little 

 over a foot high, made excellent material. 



REFERENCES. 



1. Beer, Rudolf, "The Nucleolus," Natural Science, vol. vii. (September), p. 185, 



1895. 



2. Buscalioni, L., " Osservazioni e richerche sulk cellula vegetale," Estratto dal- 



V Ann. del r. inst. hot. di Roma, vol. vii. 1898. 



3. Buscalioni, L., and R. Pirotta, "Sulla presenza di dementi vascolari multi- 



nucleati nelle Dioscoreacee," Estratto dall' Ann. del r. inst. hot. di Roma, 

 vol. vii. 1898. 



4. Campbell, D. H., " The Structure, etc., of Mosses and Ferns," p. 273 (and Fig. 143 C). 



London, 1895. 



