438 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 Zea 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, 
anid. 17 ). 
3efore 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 Zea 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 Zea, in the leaf-sheaths of 
Secale cereale (rye), the leaf-sheaths and young stems of Triticum 
vulgare (wheat), the leaf-sheaths of Hordewm sativum (barley), and 
Dactylis glomerata (cock’s-foot grass). 
In conclusion, I 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. 
REFERENCKS. 
1. Brer, Rupoir, ‘‘The Nucleolus,” Natural Science, vol. vii. (September), p. 185, 
1895. 
2. Buscauiont, L., ‘ Osservazioni e richerche sulla cellula vegetale,” Estratto dal- 
? Ann. del r. inst. bot. di Roma, vol. vii. 1898. 
3. Buscaniont, L., and R. Prrorra, ‘‘Sulla presenza di elementi vascolari multi- 
nucleati nelle Dioscoreacee,’ EKstratto dall’ Ann. del rv. inst. bot. di Roma, 
vol. vii. 1898. 
4. CAMPBELL, D. H., ‘‘ The Structure, ete., of Mosses and Ferns,” p. 273 (and Fig. 143 C). 
London, 1895. 
