Studies on the Binucleate Phase in the Plant-cell. 17 



stages were found (PI. I, fig. 12 A), and also paired daughter nuclei, 

 in telophase and in resting stages, enclosed in phragmospheres 

 (Plate I, figs. 12 B and 12 C). The nuclei, as Miss Prankerd has 

 already recorded, often exceed two in number ; I have seen a case 

 of a nucleus dividing by mitosis in a cell also including two resting 

 nuclei. I have not seen in my preparations any group or 

 "" complex " of nuclei suggesting an origin from one parent nucleus 

 by direct division, such as that figured by Miss Prankerd (I.e. fig. 5), 

 though nuclei were often seen in close contact, and more or less 

 overlapping one another. It is possible that the appearance of a 

 '* complex " may be brought about by the method of fixation used. 

 The nuclei of Morus have one or more nucleoli, each surrounded 

 by an exceptionally wide clear areola. This type of structure 

 suggests fragility, and these nuclei seem in practice particularly 

 sensitive to the action of the fixing' agent. I have found that 

 when fixed with alcohol-acetic the nuclei are perfectly distinct, but 

 chrom-acetic acid, which evidently does not suit the material, gives 

 curious results ; the individuality of the nuclei in a cell is often 

 obscured, and irregular figures are obtained which might easily be 

 mistaken for amitosis, if the comparison with alcohol-acetic 

 material did not prove them to be artefact. In well-preserved 

 material I have often seen individual nuclei of bilobed form, such 

 as those figured by Miss Prankerd (I.e. fig. 4 B), but I have found 

 no evidence for regarding them as stages in direct division. A 

 somewhat similar lobing of the nuclei observed in the young roots 

 of Stratiotes* which in a former paper I described as an indication 

 of amitosis, I now interpret differently (see pp. 19-21); I have also 

 observed the constant occurrence of neatly bilobed nuclei in the 

 young epidermis of Hemerocallis fulva, but here again the lobing 

 appears to have no connexion with direct division (see p. 15). 



Miss Prankerd suggests that at later stages the transition to 

 a uninucleate condition is probably brought about by the ultimate 

 occurrence of deferred wall formation between the nuclei of the 

 binucleate cells. I have in my preparations observed nothing 

 which suggests such an occurrence, and on general grounds I am 

 inclined to regard it as unlikely.! On comparing the basal region 

 of an opening bud gathered on May 8 with that of a long shoot 

 gathered on September 3, it was found that the total diameter had 

 increased by 80 p.c, while the number of elements on the greatest 

 diameter of the pith had increased only by about 12 p.c. This 

 shows that the increase in girth during the season's growth is 

 accompanied by relatively little cell division. I have further 

 observed that the mitosis of the pith nuclei is not always followed 



* Arber, A. (1914). 



t The occurrence of wall formation after amitosis was suggested as a possi- 

 bility in my former paper on Stratiotes (Arber, A. (1914)), but further work has 

 convinced me that this idea is untenable. 



C 



