412 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9. 
de Vries, H. 1915. The coefficient of mutation in Oenothera biennis L. Bot. Gaz. 59: 
169-196. 
EXPLANATION OF PLATES 
All figures were drawn with the aid of a camera lucida, using a Spencer high-powered 
binocular microscope, with Spencer 1.8 oil-immersion objective and loX eye-piece. They 
were enlarged by the use of a pantograph, and reduced one fourth in reproduction. Mag- 
nification 2,280 diameters, except figure 35, which is at a magnification of 1,140. 
Plate XXV 
Figs, i, 2. Resting stage before heterotypic prophase. 
Fig. 3. Parallelism, due to contraction of threads here and there, indicating ap- 
proaching prophase. 
Fig. 4. Beginning of prophase. 
Fig. 5. Network is beginning to contract to form synizetic knot. 
Figs. 6, 7. Tangential sections during early synizesis, showing disappearance of 
some threads and augmentation of others. 
Figs. 8, 9. Later synizesis. The threads becoming more uniform and thicker. 
Fig. 10. Spireme well formed. Threads near the nucleolus gorged with material. 
Figs, ii, 12. End of synizesis. Thread single and uniform except near the nucleo- 
lus, often beaded. Endonucleolus present. 
Plate XXVI 
Fig. 13. Same as last two figures. Nucleolus returning to the center. 
Fig. 14. Open spireme stage. Notice connection between the endonucleolus and 
the spireme. (See also fig. 16.) 
Fig. 15. Beginning of second contraction. Central portion of reticulum begins to 
thicken. Peripheral portion thrown into radiating loops. 
Figs. 16-18. Later stages in condensation, the central region larger, the peripheral 
threads thicker and shorter. Note position of nucleolus in figure 18. 
Fig. 19. Further stage in contraction. Note chromosome forming the distal end of 
a loop. The sides of two of the loops seem to have partially fused together. This stage 
is frequently seen. 
Fig. 20. Loops, or bivalent chromosomes, radiating from the central region. 
Fig. 21. The climax of contraction. Loops still plainly seen, however. 
Fig. 22. Second contraction knot begins to loosen. Two rings at one end and 
one at the other. 
Fig. 23. Slightly later stage. One loop at the top, two rings at the bottom of the 
figure. Usually at about this stage one or more bivalent chromosomes separate entirely 
from the rest. Note endonucleolus. 
Fig. 24. Circle of four chromosomes, to which are linked two two-linked chains, 
each link being a pair of chromosomes. 
Plate XXVII 
Fig. 25. Circle of four, with one two-linked chain attached. Three rings separated. 
Fig. 26. Circle of four, with three rings attached. 
Fig. 27. Circle of four, with one two-linked chain, and one single ring attached. 
Fig. 28. Circle of four, and five separate rings. Mid-diakinesis. Nucleolus melting. 
Fig. 29. Multipolar spindle. The circle of four unbroken. Two rings still linked 
to it. Endonucleolus still present. 
Fig. 30. Metaphase of the heterotypic mitosis. Note regularity of arrangement. 
Fig. 31. Anaphase of the heterotypic mitosis. 
