1905] SHATTUCK—ULMUS AMERICANA 217 
sixteen to twenty-four cells. Two tubes passing down the same 
micropyle were occasionally noted (jig. 20). 
The male cells lose their cytoplasm on entering the pollen tube, 
and during their journey to the embryo sac are simply elongated 
nuclei (figs. 16, 33). They were found side by side in the tube soon 
after leaving the pollen grain and were still somewhat elongated and 
very close together on entering the embryo sac, where the tube 
enlarges in a very irregular cyst-like manner (fig. 33). After enter- 
ing the sac the nuclei become spherical and begin to gather a small 
amount of cytoplasm around them. The first to enter the sac 
generally fuses with the fused polars (fig. 34), the second fusing with 
the egg. Fertilization occurs from March 29 to April t. 
ENDOSPERM. 
The endosperm begins to form soon after the male nucleus fuses 
with the fusion nucleus (fig. 34). This almost always occurs before 
fertilization, but instances were noted where fertilization probably 
occurs first (fig. 37). This variation was noted by LAND (13) in 
Erigeron, where he found sometimes the egg and at other times the 
endosperm nucleus dividing first. CouLTER and CHAMBERLAIN (8) 
also call attention to the fact that after fertilization the egg seems to 
fest for a period, while free endosperm nuclei are being formed. 
While this may be true in a majority of cases in Ulmus (figs. 35; 30), 
many instances were found which seem to be at variance with it 
(fig. 37). 
The formation of endosperm generally proceeds rapidly and takes 
Place by free nuclear divisions, the nuclei being scattered through 
the cytoplasm of the sac. These nuclei, especially in the early stages, 
mad enormous in size and multinucleolate (figs. 35, 36, 56), the nucle- 
oli being so large as to be mistaken often for nuclei in the act of fus- 
oe Mentioned by STRASBURGER (18) for Corydalis cava. ane 
Cosperm nuclei were often found to be in simultaneous division 
throughout the sac (fig. 50), and in no instance was a rudimentary 
cell-plate noted. As the development of endosperm progresses; the 
‘Ytoplasm becomes more and more vacuolate, the nuclei take a 
Parietal position and become smaller; yet throughout its existence 
t . : 
he endosperm is characterized by large multinucleolate nuclei. 
