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



the first phragmosphere seen in serial sections through an apical 

 bud occurred at a level at which the diameter of the axis, excluding 

 the leaf bases, was less than 0*5 mm. This phragmosphere 

 occurred in the rudimentary vascular zone, which was just becoming 

 distinguishable from the pith by the form of the cells and ther 

 staining power of the nuclei and cytoplasm. Binucleate cells- 

 rapidly become very numerous, and at 1 cm. from the apex the- 

 pith contains a large number of binucleate and some trinucleate 

 cells. Binucleate cells occur also, though less frequently, in the 

 cortex. The binucleate phase is somewhat protracted ; phragmo- 

 spheres have been observed at 19 cm. from the apex in a shoot 

 gathered on June 5, while occasional binucleate cells occurred in 

 the outer part of the pith at 80 cm. from the apex in a stem 

 collected on June 20 in a previous season. The change from the 

 binucleate to the uninucleate condition comes about through the 

 degeneration of one nucleus. Cells showing one normal nucleus, 

 while the other is contracted and irregular in form and stains deeply, 

 have been observed at distances of 1, 3, and 5 cm. from the apex. 

 The surviving nuclei are often bilobed, the lobes being sometimes 

 acutely pointed as in H. Nuttallii. The number of chromosomes 

 is large, and the number of nucleoli in the resting nuclei may range 

 from 1 to 8, in nuclei observed in the same section. 



5. Syringa vulgaris L. 



In the unbranched, lateral shoot of the common Lilac, Syringa 

 vulgaris* the binucleate phase is well represented. Many of the 

 pith cells are bi- or even tri-nucleate, and binucleate cells occur, 

 though less freely, in the smaller-celled cortex. These lateral 

 shoots grow to a considerable length in a single season, and their 

 interest, from our point of view, lies in the fact that they are 

 characterized by binucleate cells throughout a remarkably long 

 region. In two sets of serial sections through different apical buds, 

 the first phragmosphere was observed in each case at about • 1 mm. 

 from the apex ; while in a shoot gathered on June 19 binucleate 

 cells were still to be found in the perimeduUary zone as far as 93 cm. 

 from the apex. The binucleate cells still persist in the perimeduUary 

 zone after the inner cells of the pith have become uninucleate or have 

 lost their nuclei altogether. In the older part of the shoot, the pith 

 nuclei show signs of decadence, and there are some rather obscure 

 indications that in the case of cells with paired nuclei one may 

 degenerate more rapidly than the other. But the long continuance 

 of the binucleate phase makes this plant an unfavourable subject 

 for following out the fate of the nuclei. 



* Miss Prankerd (1916) has recorded the occurrence of binucleate cells in the 

 petiole of this species. 



