EXPLANATION OF PLATES 



For slides of sectioned material Leitz' 2 mm.-apoehromatic objective, aperture 

 1.4, together with binocular eye-piece X 15 has been applied; for iron-acetocar- 

 mine smears, Leitz' 3 mm.-apochromatic objective, together with the same eye- 

 piece. Parallel rays from a small circular source of light of great intensity were 

 sent through Leitz' achromatic condenser aperture 1.4 for illumination. 



The magnification of sectioned material is about X 2100. In the preparations 

 treated with acetocarmine the swelling of the chro^nosomes caused by the acetic 

 acid just compensates the minor magnification of the 3 mm. -objective as com- 

 pared with the 2 mm. -objective, so that all pollen mother cells reproduced appear 

 of practically the same average size. 



In the tetrads and polyads stress has been laid mainly upon number, size, 

 and shape of nuclei and cells, and the structure shown in the nuclei of these must 

 be regarded only as a kind of signature. 



PLATE 58 



Crepis aspera. 



Fixation: Carnoy-Navashin. Sectioned material. Stained in gentian violet- 

 iodine-orange G, 



Fig. 1. Zygophase; the thick chromomeres probably have been formed by 

 parallel union of pairs of chromomeres. 



Fig. 2. a. The chromatic thread opens up, showing its doubleness; b, one of 

 the four bivalents in diplophase, a little later. Seven twists of its partners around 

 each other were counted; note the chromomeres. 



Fig. 3. The four bivalents in one pollen mother cell in diplophase; the part- 

 ners still much twisted around each other. 



Fig. 4. Later diplophase; four bivalents are seen; the partners untwist but 

 the chromatic thread coils up in a spiral. 



Figs. 5-9. Diaphases (diakinesis). In all pollen mother cells 4 bivalents can 

 be counted. Note the two spiral chromatids in each chromosome in this phase. 

 Chiasmas are clearly seen in figures 5-8; in figure 6 one bivalent with a chiasma 

 is drawn outside the nucleus in order to show the underlying bivalent; in figure 7 

 one bivalent probably has two chiasmas; in figure 8 there is one clear chiasma, the 

 two partners forming a cross. Figure 9 shows different shapes of bivalents in 

 one pollen mother cell. 



Figs. 10-11. Two heterotypic metaphases; different shapes of bivalents. 



Crepis bursifolia. 



Sectioned material; fixed and stained as the preceding ones. 



Figs. 12-13. Diaphases of two pollen mother cells; different shapes of bi- 

 valents (rings, crosses). Note the short pair and the spiral chromatids. 



Figs. 14—15. Two heterotypic metaphases, each with 4 bivalents; in figure 14 

 three crosses and perhaps one rod ; in figure 15 a double J, two rings and one rod. 

 Note the short bivalent. 



[426] 



