190 BOTANY 



thin peripheral area of deeply staining material lying within the 

 nuclear membrane. It is granular in nature and not visibly 

 aggregated into a chromatin reticulum. A well-marked spherical 

 nucleolus is also present. At the initial stage of protomitosis there 

 is an aggregation of peripheral chromatin to form a ring (Fig. 72, 1) 

 round the periphery of the nucleus in close association with the 

 nuclear membrane. Simultaneously intranuclear spindle fibres 

 make their appearance, lying across the nuclear cavity at right 

 angles to the chromatin ring. The spindle is equal on both sides 

 of the chromatin plate and the chromatin appears as a line lying 

 across the nuclear cavity. The ring of chromatin now splits 

 across, forming two rings, which are apparently similar to one 

 another in size and shape (Fig. 72, 2), and the central nucleolus 

 (karyosome) elongates at the same time and lies across the 

 chromatin plate. As soon as the split becomes more evident in 

 the chromatin plate, the nucleolus becomes still more elongated. 

 The nuclear membrane at this time becomes less spherical and 

 begins to elongate in the same direction as the nucleolus. Further 

 elongation of the nucleolus leads to its spherical ends being joined 

 by a thin strip. The chromatin plates precede the nucleoli to the 

 poles of the spindle and a " double anchor " or dumb-bell effect is 

 obtained. Finally the nucleolus separates into two and the 

 chromatin j^lates become more or less completely curved round the 

 two parts. There is no clear evidence at this stage of chromo- 

 somes, or for the matter of that at any stage of protomitosis. 



The reproductive phase in these plants is separated by the 

 so-called akaryote stage. This is brought about by the gradual 

 reduction in the size of the nucleolus, which eventually disappears. 

 The chromatin within the nuclear membrane shows a gradual 

 decrease in amount, and small granules of chromatin make their 

 appearance in the cytoplasm. Finally all the chromatin passes 

 from the nucleus through the nuclear membrane into the cyto- 

 plasm. 



At the conclusion of the akaryote stage, the nucleus is reorga- 

 nised and a typically mitotic division takes place, in which appa- 

 rently there is a reduction in the number of chromosomes. This 

 mitotic division occurs immediately before spore formation. The 



