SPERMATOPHYTES— ANGIOSPERMS 



365 



I |iSi^:4?S-3^ 



B 



The Chicago chromo-acetic-osmic formula is good for the entire 

 series. Iron-alum haematoxylin, with a light touch of orange in clove 

 oil, is best for the chromatin. For general beauty and for the achro- 

 matic structures, the safranin, gentian violet, orange combination has 

 not been excelled. The photomicrographs (Figs. 129-131) illustrating 

 the series from the archespo- 

 rial cell (which, in this case, 

 is also the primary sporoge- 

 nous cell and the megaspore 

 mother-cell) to the four meg- 

 aspore nuclei will repay a 

 careful study. One more mi- 

 tosis produces the 8-nucleate 

 embryo sac, but Lilium is 

 not a good type for illustra- 

 tive purposes, since the egg 

 apparatus is not very defi- 

 nitely organized. 



For the embryo sac at 

 the fertilization stage, many 

 of the Compositae are good. 

 Senecio aureus is qujte favor- 

 able, because it is easy to 

 cut and the akenes do not 

 spread. Aster gives an ex- 

 ceptional view of the antip- 

 odal region, but is rather 

 hard to cut. Before fixing, 

 trim the head as indicated 



in Figure 132. Silphium, especially S. ladniatum, furnishes an ideal 

 view of the embryo sac. With thumbs and fingers grasp the two 

 wings of the akene and carefully split it, exposing the single white 

 ovule inside. This is rather tedious, but every ovule will yield a 

 perfectly median longitudinal section of the embryo sac, and there 

 is not the slightest difficulty in cutting. When the rays look their 

 best, the embryo sac is ready for fertihzation, or the pollen tubes 

 may be entering; as the rays begin to wither, you will find fertilization 

 or early stages in the embryo and endosperm. Sections should be 

 about 10 /i thick. 



Fig. 132. — A, head of Aster; B, pod of Capsella; 

 C, transverse section of ovary of Lilium. The dotted 

 lines show how the material should be trimmed before 

 fixing. 



