238 Nichols, A morphological study of Juniperus communis var. depressa. 



Fig. 11. Sporangium during first heterotypic division. X 285. 



Fig. 12. Sporangium after formation of tetrads. X 285. 



Fig. 13. Sporangium shortly before its dehiscence. X 285. 



Fig. 14. Cells in wall of mature microsporangium. Surface view. X?5. 



Plate IX. 



Successive stages in tetrad division of microspore mother cells. 



Fig. 15. Nucleus of mature spore mother cell. X 1280. 



Figs. 16, 17. ISTuclei preparing to enter synapsis. A pairing of the chro- 

 matic material is taking place. X 1280. 



Fig. 18. Nucleus during synapsis. X 1280. 



Fig. 19. Nucleus emerging from synapsis. X 1280. 



Fig. 20. Nucleus fully recovered from synapsis. The spirem is slender 

 and uniformly distributed, while indications of a fibrillar structure are already 

 apparent in the cytoplasm of the cell. X 1280. 



Fig. 21. Spirem shortened and thickened. Nucleus surrounded by cyto- 

 plasmic "feit". X 1280. 



Fig. 22. Thin tangential section of nucleus to show longitudinal Splitting 

 and transverse segmentation of spirem. X 1280. 



Fig. 23. Diakinesis. X 1280. 



Fig. 24. Multipolar spindle organized. X 1280. 



Fig. 25. Multipolar diarch spindle. Ohromosomes are oriented at the 

 equatorial plane. X 1280. 



Fig. 26. Bipolar spindle. Separation of daughter chromosomes about to 

 take place. X 1280. 



Fig. 27. Same stage as that shown in the preceding figure. Section 

 cut perpendicular to axis of spindle and showing twelve chromosomes. X 1280. 



Fig. 28. Anaphase of heterotypic division. A longitudinal fission of 

 the daughter chromosomes apparent. X 1280. 



Fig. 29. Late anaphase, showing the massing together of the chromo- 

 somes at the poles. X 1280. 



Fig. 30. Late telophase of heterotypic division, showing reconstruction 

 of daughter nuclei. X 1280. 



Fig. 31. Resting daughter nucleus of heterotypic division. X 1280. 



Fig. 32. Metaphase of homotypic division. Separation of the grand- 

 daughter chromosomes about to take place. X 1280. 



Figs. 33, 34. Telophase of homotypic division. First indications of a 

 thickened wall enclosing the protoplasm of the mother cell. X 1280. 



Fig. 35. The same. Nuclei connected by cytoplasmic fibrils. Thickened 

 wall well developed. X 1280. 



Plate X. 



Figs. 36 — 39. Development of microspore. 



Fig. 36. Reconstruction of granddaughter nuclei and formation of cell 

 plates. X 1280. 



Fig. 37. Laying down of walls separating nuclei of tetrad. X 1280. 



Fig. 38. Separating walls have swollen, and have become continuous 

 with the thick, enveloping wall. X 1280. 



Fig. 39. Mature microspore packed with starch grains. X 1280. 



