18 Transactions of the Society, 



by the formation of a phragmosphere ; cases of progressive wall 

 formation have also been noticed sufficiently frequently to account 

 for the small amount of cell division that occurs in the later stages 

 of development of the pith. These facts taken together seem to 

 eliminate the possibility of subsequent wall formation occurring: 

 between the nuclei of binucleate cells — unless these walls are 

 entirely confined to horizontal planes and are thus invisible in 

 transverse sections. 



Morns nigra does not seem to be a particularly favourable plant 

 for studying the fate of the nuclei. My observations lead me to 

 believe that in many cells of the pith the binucleate condition i& 

 very persistent, and that both nuclei perhaps survive in some- 

 of the cells, as long as these elements remain nucleated at all. In 

 certain cells, on the other hand, I have observed, in September,, 

 appearances indicating that one nucleus becomes moribund while 

 the other remains normal. I am thus disposed to think that the 

 transition from the binucleate to the uninucleate condition comes- 

 about in the same way as in the other plants here described, but I 

 do not wish to lay much stress upon this point, as my results in 

 the case of Morus nigra, if taken alone, are by no means conclusive. 



10. Eippuris vulgaris L. (PL I, figs. 17-20), and Modea 

 canadensis Michx. (PI. I, figs. 29-32). 



In a paper published in 1914 Dr. McLean* has described 

 amitosis giving rise to binucleate cells in the parenchyma of the 

 young stem cortex of species belonging to nine different genera 

 of flowering plants and one fern. Since the observations made by 

 Mr. Beer and the present writer on a number of other genera have^ 

 as we have already shown, indicated that the general origin of the 

 binucleate condition is through karyokinesis, I have re-examined 

 two of Dr. McLean's cases. As Hippuris vulgaris is the species 

 to which he devotes the greatest attention, and in which he figures- 

 the binucleate cells, I chose this plant as a Dicotyledon, and Modea 

 canadensis as a Monocotyledon, for investigation. 



In my material of Jlipptcris vulgaris the cortex showed 

 numerous cases of paired fusiform nuclei as figured by Dr. McLean. 

 But I have found no evidence at all that these nuclei arise through 

 direct division, and I have seen none of the stages of " longitudinal 

 fission " which he describes. But on the other hand I have seen 

 all stages of karyokinesis (e.g. PL I, figs. 17 and 18), and, finally, 

 pliragraospheres with the paired nuclei at rest (PL I, figs. 19 and 

 20). The nuclei are exceptionally small, as will be recognized on 

 comparing PL I, figs. 17-20, which represent them, with those, for 



♦ McLean, R. C. (1914). 



