P seudotsuga Douglasii. 1 79 
Sokolowa, M lle C. (’90) : Naissance de 1’endosperme dans le sac embryonnaire de quelques 
Gymnospermes. Bull. Soc. Imp. Nat. Moscou, 1890 (1891), p. 446. 
Stopes, M. C., and Fujii, K. (’ 06) : The Nutritive Relations of the surrounding Tissues to the 
Archegonium in Gymnosperms. Beihefte zum Bot. Cent., Bd. xx, Heft 1, 1906. 
Thomson, R. B. (’05) : The Megaspore-membrane of the Gymnosperms. Univ. Toronto Studies, 
Biol. Series, iv, 1905. 
EXPLANATION OP' FIGURES IN PLATES XII, XIII, AND XIV. 
Illustrating Dr. Lawson’s paper on Pseudotsuga Douglasii . 
All figures were drawn with the aid of the Camera Lucida. 
Fig. 1. A cross-section of a microspore before pollination, showing the generative nucleus, the 
tube-nucleus, and two degenerating prothallial nuclei. April 2. 
Fig. 2. A longitudinal section of a young pollen-tube, showing the body-cell ( B ), the stalk cell 
(.S’), and two degenerating prothallial cells (P', P''). April 28. 
Fig. 3. A longitudinal section of an ovule some time after pollination, showing the upper 
expanded region of the micropyle with a ‘ stigmatic ’ surface, and in which two germinating 
microspores are seen. April 28. 
Fig. 4. The same at a later stage, showing numerous pollen-tubes growing from the distal 
or expanded end of the micropyle towards the nucellus. May 23. 
Fig. 5. A pollen-tube soon after the division of the generative nucleus, showing the stalk- and 
body-nuclei and the tube-nucleus. May 3. 
Fig. 6. A pollen-tube at a later stage, showing the body-nucleus advancing towards the tip of 
the tube. May 23. 
Fig. 7. The body-cell as it lies near the tip of the pollen-tube. May 23. 
Fig. 8. The same at a later stage. The body-nucleus is preparing for division. May 23. 
Fig. 9. A longitudinal section through the sporogenous region of a megasporangium, showing 
one large functional megaspore which has already germinated, and two very small abortive mega- 
spores. A distinct layer of large cells, the tapetum, surrounds the megaspores. March 30. 
Fig. 10. A longitudinal section through an ovule, showing the functional megaspore and the 
tapetum considerably enlarged. The young prothallium within the megaspore consists of a thin 
parietal layer of cytoplasm in which numerous free nuclei are distributed, and a large central vacuole. 
A distinct megaspore-membrane is already quite visible at this stage. April 28. 
Fig. 11. The young prothallium shown in Fig. 10 more highly magnified. April 28. 
Fig. 12. A small portion of the parietal layer of cytoplasm, showing the relative size of the 
free nuclei. April 28. 
Fig. 13. A portion of a young prothallium at a later stage, showing the formation of the first 
or primary prothallial cells. These primary cells have no walls on the inner side exposed to the 
central vacuole. The megaspore membrane shows a considerable increase in thickness at this stage. 
April 16. 
Fig. 14. A portion of the periphery of a mature prothallium, showing the structure of the 
megaspore membrane at this stage. May 23. 
Fig. 1 5. A small portion of a section of a very young prothallium, showing the formation of the 
primary prothallial cells. The section is taken parallel to the inner or open surfaces of the primary 
cells. April 16. 
Fig. 16. A longitudinal section through the lower or sterile end of a young prothallium, showing 
the primary prothallial cells very much elongated, and growing inward and encroaching upon the 
space occupied by the central vacuole. April 16. 
Fig. 17. A section through a portion of the archegonial end of a mature prothallium, showing 
the gradual tapering off of the megaspore membrane in the region of the archegonia, and the entire 
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