4 MINNESOTA BOTANICAL STUDIES. 



four-celled stages were not encountered. On the other hand the 

 stage where four nuclei are closely grouped at either end was often 

 found (Fig. 12, PI. I). The mature embryo-sac varies considerably 

 in form and size but is generally twice as long as wide and oval in 

 outline. The two polar nuclei behave in the usual way, approach- 

 ing and fusing near the middle of the sac (Fig. 17, PI. I). 



In the micropylar end of the embryo-sac are found at this stage 

 of its development a number (4-6) of more or less darkly staining, 

 irregular bodies. They lie closely grouped just outside the four 

 nuclei and are evidently the remnants of the other germinating 

 megaspores (Fig. 12-13, P^- I)- They persist only for a short 

 time and disappear when the fertilized ovum begins to divide. The 

 presence and position of these bodies seem to indicate that the inner- 

 most megaspore is the one that gives rise to the embryo-sac. The 

 egg and synergids enlarge and elongate and the usual vacuolate 

 character of these cells becomes evident (Fig. 13, PI. I). 



In the process of fertilization the pollen tube makes its way 

 through the micropyle and penetrates the nucellar tissue, some- 

 times in a direct manner, at other times more or less irregularly, 

 and enters the embryo-sac (Fig. 33, PI. III). In no case, however, 

 have the sperm cells been observed nor actual nuclear fusion seen. 

 Presumably it takes place in the usual way. 



The ovum begins to enlarge somewhat and the first division in 

 the young embryo is transverse to the long axis of the embryo- 

 sac (Fig. 14, PI. I). Other divisions follow in the same plane as 

 the first, and a row of three to four cells is built up before anticlinal 

 walls appear (Fig. 15, PI. I). These latter proceed from the tip 

 towards the base. The synergids remain during the first stages of 

 embryo formation but soon begin to break down and disintegrate 

 (Fig. 14-15, PI. I). 



In the development of the endosperm a brief period of free cell 

 formation (Fig. 14-16, PI. I) is followed by the appearance of cell 

 walls, whereby a tissue is built up throughout the embryo-sac (Fig. 

 20, PI. I). The endosperm nuclei as they are formed distribute 

 themselves more or less irregularly throughout the embryo-sac and 

 the walls appear about simultaneously between all the nuclei. The 

 endosperm grows rapidly and soon begins to encroach upon the 

 remnants of the nucellar tissue (Fig. 37, PI. Ill) then upon the inner 

 integument (Fig. 35, PI. Ill), and finally begins to push back into 



