570 TRANSACTIONS OF SECTION K. 



sporangia have not yet been clearly seen. We are now able 'to go a short dis- 

 tance towards splitting up the old artificial genus Cladophlebis. 



Several plant-remains new to Yorkshire have been found, such as Neocalamites 

 at Whitby, Marattiopsis at Marske, and a structure resembling the sporocarps of 

 a Marsilia at Gristhorpe. It seems probable that future work will yield many 

 further interesting results. 



5. A Petrified Jurassic Plant from Scotland. 

 By Professor A. C. Seward, F.R.S. 



This paper dealt with the structure of a petrified Williamsonia collected 

 by Hugh Miller in North-East Scotland, and figured by him in the 'Testimony 

 of the Rocks.' The specimen, of which sections have been cut by permission 

 of the Director of the Royal Scottish Museum, Edinburgh, consists of a central 

 conical axis bearing immature interseminal scales and seeds, the whol§ being 

 enclosed by linear bracts bearing numerous unicellular hairs. The structure of 

 this fossil will be fully described in a forthcoming paper. 



6. A Contribution to our Knowledge of the Formation of Calcareous Nodules 

 containing Plant Remains. By Miss T. Lockhart, B.Sc. 



Three boulders from the calciferous sandstone of Pettycur had been entirely 

 cut into thin serial sections in the search for a minute object, and it was thus 

 possible to trace the position, in a block, of any particular plant. 



Mctachpsydropsis duplex and Botryopteris antiqua were chosen, as they pre- 

 sented a contrast between a large and a small plant. The clear delimitation of 

 even the smallest fragments points to mechanical fracture subsequent to immer- 

 •sion, and the parallel position of plant remains in the boulder further indicates 

 the agency of water currents. 



The results confirm Dr. Gordon's views of thermal pools as the actual site of 

 petrifaction. 



7. Nuclear Osmosis as a Factor in Mitosis. 

 By A. Anstruther Lawson, D.Sc, F.R.S.E. 



A study of the microspore mother-cells of Disporium, Gladiolus, Yucca, 

 Ilcdera, and the vegetative cells in the root-tip of Allium, has revealed a series 

 of stages in the development of the mitotic spindle which has never before been 

 recorded. The new stages that have been discovered are to be found in the 

 prophase, immediately preceding the organisation of the equatorial plate, and 

 concern the fate of the nuclear membrane. The interpretation of these stages 

 has thrown a new light on the process of mitosis, and necessitates a revision of 

 the accepted views of nuclear phenomena. Contrary to the generally accepted 

 view, it has been found that the nuclear membrane does not break down or 

 collapse at any period during spindle development, but behaves as one would 

 expect a permeable plasmatic membrane to behave under varying osmotic 

 conditions. 



The nucleus is regarded as an osmotic system, and its membrane constitutes 

 an essential element in that system. It is a fact of common knowledge that the 

 chromatin changes both in quantity and form some time before the metaphase. 

 The chromatin must increase in quantity because the same amount is present 

 with each mitosis. It changes in form from the finely divided condition repre- 

 sented in the reticulum and spireme to the more compact and homogeneous form 

 of the chromosomes. It would seem that these changes are in some way 

 correlated with a variation in the osmotic relations of the karyolymph. At any 

 rate, a gradual diffusion of the nuclear sap immediately follows these changes 

 in the chromatin. 



A series of stages has been found showing beyond much doubt that closely 

 following the organisation of the bivalent chromosomes there takes place a 

 gradual diminution in the volume of the nuclear vacuole. It is believed that 

 the karyolymph gradually diffuses by exosmosis into the cytoplasm. Through- 



