276 



Methods in Plant Histology 



One more mitosis produces the 8-nucleate embryo-sac, but Lilium 

 is not a good type for illustrative purposes, since the egg apparatus 

 is not very definitely organized. 



For the embryo-sac at the fertilization stage, many of the Corn- 

 posit ae are good. Senecio aureus is quite favorable, because it is 

 easy to cut and the akenes do not spread. Aster gives an exceptional 



view of the antipodal re- 

 gion, but is rather hard to 

 cut. Before fixing, trim the 

 head as indicated in Fig. 99. 

 Silphium, especially S. lacini- 

 atum, furnishes an ideal view 

 of the embryo-sac. With 

 thumbs and fingers grasp 

 the two wings of the akene 

 and carefully split it, expos- 

 ing the single white ovule in- 

 side. 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 fer- 

 tilization, 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 ju thick. 



The Ranunculaceae, especially Anemone patens var. Wolfgan- 



giana, show a rather large, broad embryo-sac, with highly organized 



egg apparatus and antipodals. Sections should be 10 to 20 ju thick. 



For general views of the embryo-sac, the safranin, gentian-violet, 



orange combination is recommended. 



Fertilization .--The later stages cut to show the mature embryo- 

 sac will often show fertilization. The male and female nuclei almost 



FIG. 99. A, head of Aster; B, pod of Cap- 

 sella; C, transverse section of ovary of Lilium. 

 The dotted lines show how the material should 

 be trimmed before fixing. 



