Study of Reproduction in the Genus Acer 135 



Fig. 3. Leptonema entering synapsis as a network. 



Fig. 4. Mid-synapsis. 



Fig. 5. Looped pachynema threads leaving synapsis. 



Fig. 6. Ditto. 



Fig. 7. Hollow spireme. 



Fig. 8. Contracting hollow spireme passing into strepsinema. 



Fig. 9. Early strepsinema. 



Fig. 10. Strepsinema, thread breaking up. 



Fig. 11. Strepsinema, thread largely broken into chromosome segments. 



Fig. 12. Ring-like chromosome segments. 



Fig. 13. Ditto. 



Fig. 14. Breakdown of the nuclear membrane, strands of cytoplasm 

 traversing the cavity. 



Fig. 15. Polar view, heterotypic metaphase, showing thirteen chromo- 

 somes, haploid number. 



Fig. 16. Side View, heterotypic anaphase. 



Plate VII — Stages in Reduction Divisions, Pollen Formation in Acer negundo 

 (Figs. 17-26), Acer rubrum (Figs. 28-30), and Pollen Grain of Acer 

 saccharinum (Fig. 27). All except Fig. 27 reproduced X2250. 



Fig. 17. Multipolar spindle, early with large nucleolus. 



Fig. 18. Multipolar spindle, later, with smaller nucleolus (in center). 



Fig. 19. Side view, telophase of heterotypic division. 



Fig. 20. Polar view, telophase of heterotypic division, showing thirteen 

 chromosomes. 



Fig. 21. Late telophase, showing formation of nucleolus, the large body 

 in center of nucleus. 



Fig. 22. Nucleus during interkinesis. 



Fig. 23. Prophase of homotypic division. 



Fig. 24. Polar view, breakdown of nuclearmembrane, homotypic divi- 

 sion, nucleolus evident. 



Fig. 25. Multipolar spindle, homotypic division, showing the remains of 

 the homotypic spindle, and the nucleoli near the periphery of the cell. 



Fig. 26. Homotypic division, side and polar views, showing remains of 

 heterotypic spindle and thirteen chromosomes in the polar view of the 

 metaphase. 



Fig. 27. Pollen grain, mature state, with tube and generative nuclei, 

 Acer saccharinum. X950. 



Fig. 28. Strepsinema, Acer rubrum. 



Fig. 29. Side view, heterotypic metaphase, showing chromosome pairs 

 and polar position of the dense mass of cytoplasm. Seventy-two 

 chromosome form. 



Fig. 30. Polar view, heterotypic metaphase, showing lateral position 

 of the dense cytoplasmic mass. Shows count of seventy-two chromo- 

 somes. 



Plate VIII — Tapetal Cells of Acer negundo (Figs. 31-33). Embryosac and 

 Embryo Formation in Acer rubrum (Figs. 34-41, 43-49). Embryosac 

 and Embryo of Acer saccharinum (Figs. 42, 50). Reproduced X200, 

 except as indicated. 



